Usually, 2 hrs of activity time is around 4 months old. However, all babies are different and some can last longer than others. So, if you feel like 2 hrs is a good amount of time, then go with it. You know your baby. If you feel like 2 hrs. is a little too long, then trim it back by 5-10 min and see if that works better. Just watch his cues, which it sounds like you are doing. I can't always go by yawns either with my son. I, like you, wait for the whining and he goes down easier. Every baby has different ways of letting us know what they need. It sounds like you are in tune with your son!