From UKBA website: [When should you apply?
You must be in the UK when you make your application, and you must apply before your current permission to stay expires. We encourage you to apply 28 days before the expiry date, but you can choose to apply earlier than this.]
In my case, I applied less than 28 days. I submitted my requirements by post May 7, and my visa expiration date is June 1.
For more information about FLR(M) application, click this link.
http://www.ukba.homeoffice.gov.uk/vi...pplying-in-uk/
Hope it answers your questions. :-)
