Yeah, I agree with Andrea.
Usually when my lo takes a 45 min nap, I tend to freak a little. She has historically been a great napper, and when she does the 45 min nap thing, I *know* she hasn't napped long enough, and I try to fix it. Usually I just end up extending "A" time, which does the trick nicely. But occasionally she goes through phases of short naps no matter what I do, so I get to the point where I tell her "If you want to be up, no fussing because you're tired!" and go about our day (because she refuses to go down again until it's nap time again or bedtime). Like she really understands that...but it does seem to work somehow, and I chalk it up to some developmental phase coming on.
I've also done a lot of research into sleep habits, and for awhile I was obssessed with it. Now I'm obsessed to a certain degree, but I don't panic about it quite as much as I use to. After reading through a lot of posts and watching responses, learning from many of them, comparing it to the research, and comparing it to my lo's behavior, I find that more and more while research is a great guideline when diagnosing a problem and fixing it, babies are vastly different enough that sometimes you have to throw guidelines out the window and just go with your gut, because your lo's problem just doesn't fit into the "research mold".
You know that your lo has had enough sleep by how he/she seems to be when they wake from a nap. If they're crying, then chances are they didn't sleep long enough. If they play happily, then they're fine. While it's always a good idea to at least try and convince them to sleep a little longer so they don't get fussy after only an hour or so of being awake after a short nap, even if they wake happy, as long as they're getting rested most days in your opinion based on their behavior most days, one odd day or two isn't going to totally throw them off.
If your lo has been historically a short napper no matter what you do, then chances are a) it will iron itself out by 5-7 months and they will magically become a great napper, or b) they'll always be a short napper, and they fall into that 18% category that Andrea was talking about.
Anyway, just some thoughts from my experience.
*HUGS*
Sonya =P