BAA publish customer service stats on their web site for each airport and in regard to baggage waiting times the scoring is overall rather than at specific times, so if you arrive at Stansted most times of the day the scoring indicates reasonable waiting times.
It's a while since I've experienced a late evening arrival at Stansted and it wasn't a pleasant experience then. Too many flights and pax arriving in such a short space of time resulting in no one on stand to meet the aircraft so a delay disembarking, once off the aircraft joining the massive queue at immigration which was so long that travelators had to be switched off for health and safety reasons so people weren't queing on a moving walkway. Then to baggage reclaim -if it's after midnight then it's possible the late shift have gone home and leaving just a few nightshifters to deal with the many bags that need offloading from several aircraft. Also it's possible the staff sickness rate increases on Saturday nights leaving even fewer people to do the handling.
Lengthy baggage waiting times during the late evening at Stansted do occasionally occur for these reasons and if several flights arrive a little later than scheduled (after midnight) then this doesn't help.