I usually only cross from Poipet into Rongklua in later pm. Have had a variety of experiences. Sometimes not too many people.

This time I might arrive to Poipet 9 or 10am, with a crossover from there. Are the lines usually heavy going into Thailand, or not so much? When is the peak time(s) for heavy traffic?

i have walked across 2 times at about that time, and both times the queues were not so bad...... i would say i waited about 10 - 15 minutes at the thai immigration point where you receive your stamp

It is crossing on foot.

It's totally unpredictable, but unless there bussloads of tourists coming, it doesn't take more than 10-15 minutes.

It is crossing on foot.

+1