When my DD was a newborn, we'd comment on how long she was napping, sometimes up to 3 hours at a time. Then around 2 months of age, she started taking 20-40 minute naps and it drove me insane because I was getting nothing done. That's when I found this website and started doing everything suggested (i.e. darkened room, white noise, swaddle, holding through jolts, etc) and really nothing helped. I finally decided not to obsess and try to enjoy the rest of my maternity leave. Then when she was around 4 months old (just in time for me to return to work, lol), she started taking longer naps in her darkened room, 2-3 sleep cycles (so 1 hr 20 min to 2 hrs).
Shortly after that, she started daycare and resumed to taking 40 minute naps (it's like she doesn't know how to transition between sleep cycles in a less than perfect environment). We decided there is nothing we can do about the daycare situation as she was in a noisy nursery with 8 other babies. But at home, she continued to take longer naps.
Even today at 14 months, if we're out and about, she'll nap for only 40 minutes at a time in her stroller or carseat. At daycare, she'll also wake up at the 40 minute mark and they have to pat her back down to finish out the nap. Fortunately naps at home in her darkened room are still 1 hr 20 min - 2 hours, unassisted (i.e. no patting down, etc).
So this is maybe a mixed success story??