Having switched to another carrier and FF-program (and being very happy with that) I'm now in the situation where I have some residual FB miles which will expire this last Sep, despite my efforts to burn the miles in time.

It's not a whole lot of miles left, but when it's promo time they could net me some houses. My question: how fast are the miles actually pulled, and what kind slack will they cut you if you take a qualifying flight sometime after the formal expiry? Taking a flight now in Sep looks tight, how quick need I be before it's completely niet mogelijk to reinstate expired miles?

Any experiences out there?