Under 3 months of age, I don't recommend stretching the awake time. Right now, I would concentrate of his awake time--keeping it short, some newborns can only handle a brief period of awake time and will overtire quickly. As you mentioned, every child is different, so it's hard to put a time limit on it. But, right now, I would look at his day as awake time and sleep time. Catch him on the first yawn.. especially if his second nap is a harder one for him to go down.
It takes time, practice and patience. It will come. Don't focus on the time, just focus on his body language and cues.