I hate saying this as it's such a non-answer, but it really depends on your LO. Some babies are great if you tuck them in at the first yawn, with others it's way too late already. With both my dds if I wait until they yawn it's already too late and they're really hard to get down as they're overtired. The first few months are tough, once you think you know their schedules, they get a little older, can stay up a little longer and everything is different. If putting him down at the first yawn is working for you, then I would stick with it. If you need to extend activity time, try going into his room, dimming the lights, and just playing very quietly with him. My nearly 4 month old dd loves it when I sing songs to her (with lots of facial expressions) -- though it he starts looking away from you, he's probably overstimulated. Try showing him a small toy (though not too small), encourage him to touch it, grab it (though he won't be able to just yet) -- touch his hands with it, things like that. If he "talks" to you, talk back with a normal conversational rhythm. Try a little foot massage (my dd LOVES this, you should see the smiles!).
With my dd, I put her down for a nap when I see her start to turn away from more stimulating things (turning away from the window to stare at the couch), sometimes I notice that she's moving around less -- she also rubs her face with her hands and tries to suck on her fingers more. You'll find through lots of observation what your LO's sleepy cues are, so be on the lookout. It also helps if you write everything down -- how long the A time was, what sort of cues you were seeing, how well he went down for a nap -- that way you'll know what's working and what isn't (and what cues are truly the sleepy cues you're after).
Hope some of that is helpful!
