For us, it was right around 5/6 months that I didn't have to help him extend and he still took 1.5 hour long naps the majority of the time. Before that, we'd have times that he wouldn't need help, but he usually did. I was dissappointed because I thought it was a matter of just getting them out of the habit, but for some babies, it is truly something they just have to outgrow and their brain usually matures so they can stay asleep through the jolts around 4-6 months. So your LO might make it before he starts daycare since he's right at that age.
I'd keep working with it though as long as you can. I sometimes wonder if DS would have outgrown the 45 min. naps no matter what I did, but I know DD took a LOT longer to take good naps (I think she was around a year before it because regular), and I didn't do as good extending hers, so maybe that was the key. Plus it helps them get enough sleep. . .
~Kim