Don’t know the exact number of days but its a diminishing rate of FF generated after a week or so same for the next lowering of “FF%” generated after that (think it was a month that this hits 0%).
I do know in the beginning it was a month or so that the beacon started destroying the coin that was in the beacon.
But if the destruction of coin in a beacon still happens after a certain time i do not know.
(Seeing that this might have changed to when they changed the FF from Payment by visitor to generation).
No expert 
just a bit that i know