hi there
2 hours sounds like a pretty long stretch for the 'average' 9 week old baby... (mine was the exception and was awake all day!!)... but even now at 7 months there are often times when after 2 hours she is tired... so i wouldn't worry exactly how long your bubs has been awake before its time for sleep again, its great that you can tell when he/she is ready for sleep. if you think of it this way, you and i, we don't get up and go to bed at the exact same time each day, after the exact same duration, some days we might need more sleep than others, others we may be fine to go to bed a bit later... same with bubs, they are going to be different each time.... some days they may go an hour between naps, others 2 or 3 (as they get older).... i think its just best to watch for their cues as to when its time for sleep, and keep the clock at the back of your mind, if u know what i mean...
sounds like you have been blessed with a great little sleeper - enjoy it while its happening you never know whats around the corner :-)