I once stayed at the AMS BA lounge from 11am to 6pm, getting some work done in a quiet corner.

The lounge attendant was quietly skeptical on entry — "sir, you are aware your flight is in the evening?". But there was no mention of this long, long visit being against policy; just, perhaps, common sense. (This was before the 2014 AMS lounge renovation; it was a bit dim.)

Not the worst place I've done work, but not the best. Usually there are better choices than to spend many hours enclosed in a small room.