I too have never seen a long check-in queue at AMS for BA. The last 2 times (including a Friday afternoon peak time of 15:00 4 days ago) it was virtually deserted and both Y and CE having only 2-3 people in front of me. As UKtravelbear posted, the main problem at AMS is the exit passport control queues and these can be very unpredictable. Sometimes it is like a zoo, sometimes deserted.

3 weeks ago, coming through AMS on a Thursday lunchtime it took me 30 or so minutes to get through the departing PP check, 4 days ago there were zero queues at either the security scanners or exit passport control. It is the same at most airports around the world - they are often so variable depending on the day of the week, the time of day, how many staff might be off-sick, whether a passport scanner is out of service etc.YMMV.

Bear in mind that AMS is a huge airport and the "D gates that BA use are quite a trek from when you enter airside at the security / passport check. Depending on your walking pace, anything from 15-25 minutes to walk there.