on BA usually everything closes about 20 min before landing. About 10 min before that point, the captain says...blah blah blah...I"ll be turning on the sign in xx min. So you have till the sign goes on to ask for drinks/bathroom/etc.

Usually about 5 min after the seatbelt sign is on, crew told to take seats for landing. Though I have seen this (literally) as few as 3 min before landing.

On AA, AF, DL and NW I've had them bring drinks even as we were decending, usually close to the time of "crew pls be seated".