You have to make sure to apply for your FLR 28 days before your fiancé visa expires....and they won't take it against you if your fiancée visa has already expired and you are still waiting for the result of your FLR application.
My situation was like this: I sent off my documents for visa application 14 days before it expired. I was very worried of the outcome but thank goodness they grant it quickly ( I'm not saying here that you should submit yours less than 28 days ) as what I did was a gamble and isn't sensible at all.
If you are worried about the "long waiting" result of the application by post, apply for a premium service which is a personal appearance , although you have to pay extra for that but it guarantees you to receive your result within a week of the application not months, some even been granted their visa same day of the appointment....