Andy, this is commonly known as "Intervening Devotion"
There's no limit as such, but UKBA have declared they will place more emphasis on the needed evidence that the relationship is genuine and subsisting.
UKBA has admitted that currently there is only limited guidance about how this assessment is made. It's often down to each individual ECO.

Within the "new changes" UKBA have stated their intention to have clear guidance issued to the ECO's in order to ensure a consistent approach.


Here's what UKBA say are key factors:-

FACTORS WHICH MAY BE ASSOCIATED WITH A GENUINE RELATIONSHIP
These factors include:
• The couple are in a current, long-term relationship and are able to provide satisfactory evidence of this.
•The couple have been or are co-habiting and are able to provide satisfactory evidence of this.
•The couple have children together (biological, adopted or step-children) and shared responsibility for them.
•The couple share financial responsibilities, e.g. a joint mortgage/tenancy agreement, a joint bank account and/or joint savings, utility bills in both their names.
•The sponsor and/or applicant have visited the other’s home country and family and are able to provide evidence of this. (The fact that an applicant has never visited the UK will not be regarded as a negative factor, but it is a requirement of the Immigration Rules that the couple have met in person).
•The couple, or their families acting on their behalf, have made definite plans concerning the practicalities of the couple living together in the UK. In the case of an arranged marriage, the couple agree with the plans made by their families.

FACTORS WHICH MAY BE ASSOCIATED WITH A NON-GENUINE RELATIONSHIP
The factors which may prompt additional scrutiny of the application include:
•If the marriage or civil partnership took place in the UK, a report – of a suspected sham marriage or civil partnership – was made by the registration service under section 24 of the Immigration and Asylum Act 1999.
•The applicant or sponsor makes a public statement that their marriage is a sham. An application can be refused on the basis of such a public statement alone.
•The applicant or sponsor makes a public statement (not in confidence) that they have been forced into marriage. An application can be refused on the basis of such a public statement alone.
•A sibling of the sponsor or applicant has been forced into marriage.
•The applicant, sponsor or a family member of either is or has been the subject or respondent of a forced marriage protection order under the Forced Marriage (Civil Protection) Act 1997 or the Forced Marriage etc (Protection and Jurisdiction) (Scotland) Act 1999. An application can be refused on the basis alone of a current order involving the applicant or sponsor.
•There is information from a reliable third party (e.g. the Forced Marriage Unit, police, social services, registration service or a minister of religion) which indicates that the marriage is or may be a sham marriage or a forced marriage. (It may not be possible for this information to be used in any refusal notice). The fact that a third party indicates that in their opinion a marriage, partnership or relationship is genuine must not be afforded any weight.
•The applicant or sponsor does not appear to have the capacity to consent to the marriage, partnership or relationship, e.g. owing to learning difficulties, and independent evidence, e.g. from a social services assessment, has not been provided to confirm that such capacity exists.
•There is evidence of unreasonable restrictions being placed on the applicant or sponsor, e.g. being kept at home by their family, being subject to unreasonable financial restrictions, attempts to prevent the police or other agencies having reasonable, unrestricted access to the applicant or sponsor.
•Failure by the applicant or sponsor to attend an interview, without reasonable explanation, where required to do so to discuss the application or their welfare, or seeking to undermine the ability of the UK Border Agency to arrange an interview, e.g. by unreasonable delaying tactics by the couple or a third party.
•The couple are unable to provide any information about their intended living arrangements in the UK or about the practicalities of the applicant moving to the UK.
•The circumstances of the wedding ceremony or reception held or planned, e.g. few details, no or few guests, and/or no significant family members present.
•The couple are unable to provide accurate personal details about each other (e.g. name, age, nationality, employment, parent’s names and place of residence), provide inconsistent evidence, appear to have been coached on information about the other party rather than offer information spontaneously or do not have a shared understanding of the core facts of their relationship, e.g. how and where they met for the first time.
•The couple are unable to communicate with each other in a language understood by them both.
•There is evidence of money having been exchanged for the marriage to be contracted (unless this is part of a dowry).
•There is a lack of appropriate contribution to the responsibilities of the marriage, partnership or relationship, e.g. a lack of shared financial or other domestic responsibilities.
•Matrimonial co-habitation is not maintained (except where one party is legitimately working or studying away from home) or there is no evidence that they have ever co-habited since the marriage.
•The applicant is a qualified medical professional or has worked as a nurse or carer, and the sponsor has a mental or physical impairment which currently requires medical assistance/personal care in their own accommodation.
•The sponsor has previously sponsored another partner to come to or remain in the UK.
•The sponsor has previously been sponsored as partner to come to or remain in the UK (i.e. the sponsor has obtained settlement on this basis) and that marriage, partnership or relationship ended shortly after the sponsor obtained settlement. This excludes circumstances where the sponsor is a bereaved partner, or where the sponsor obtained settlement on the basis of domestic violence perpetrated by their former partner.
•If the sponsor was married to or in a partnership with the applicant at an earlier date, married or formed a partnership with another person and gained settlement in the UK on that basis, and is now sponsoring the original partner to come to or remain in the UK.
•The past history of the sponsor and/or the applicant contains evidence of a previous sham marriage or forced marriage, or of unlawful residence in the UK or elsewhere.
•The applicant has applied for leave to enter or remain in the UK in another category and been refused.

One of the safest ways to demonstrate this "intervening devotion" is by showing on-going physical contact. It's always a good idea to make a visit to Philippines shortly before making the visa application.

Apart from that, the evidence you'll need to submit really depends just how long you've been apart. But chat logs, e-mails, letters/postcards (with postmarks), evidence of phone contact etc will all be very useful.
Remember to keep good examples that indicate time.
Also any evidence of joint finances and/or financial support, remittances, bill payments etc

Looking at the above UKBA factors which may be associated with a non-genuine relationship, try to pick some evidence that would counter those factors.

Once the ECO guidance is published there should be a much better understanding of what's needed.

Hopefully this will help a little for the time being.