In my experience, anything from 20-35 minutes means OT, because it's not a full sleep cycle. My LO tends to wake at 35 also, as it's when she's just starting to come out of deep sleep.
As for nap extensions, I really think any intervention from me is FUTILE! I just listen, via the monitor, when she wakes and listen to see if she's fully awake and upset or still sounds sleepy and might resettle. If she doesn't go back to sleep within 5-10 minutes on her own, it's not gonna happen! I think the magic is in the A times.
But, the problem with that, as ElsMom has pointed out, is that is isn't consistent from nap to nap. So, I try to follow cues (which are much more obvious in the morning when she's not OT). If I don't see anything, I just go with the clock. If she fights too hard, like after 10 minutes, I get her up for another 10 before trying again. I'd rather enjoy her than fight her to sleep at this point. I'm confident that she knows how to settle herself and will do so when she's ready to sleep. But, she's 6 months old now--that's not something I would've said at 10 weeks.
More and more, I'm coming to the conclusion that I need to do what suits my LO--and me! In this country, we expect babies to nap independently for long periods of time. I was reading a blog about an American living in China, and she commented on how their babies only take catnaps in their moms' arms as they go about their day. And I can't believe that millions of babies are not getting their physiological needs met and that this will somehow stunt their growth/development.
Sleep training is just that. We want to take our babes to sleep in a way that suits us--and that doesn't have to mean 2 hr.. naps.
For now, I'm happy as long as she's happy. If she's rested (whatever the nap time), I find that she can place independently and that I am somewhat free to get things done