My LO goes through phases of good/bad napping. They seem to be affected by all sorts of things (teething, developmental milestones, colds...). However, when they were/are going well (and if I remember rightly), at around 3 months she used to wake at 45 mins and then I could re-settle her for another 45 mins with shh-patting and at around 4 1/2 to 5 months she started resettling herself and napping through the 45 minute mark.
She still does some 45 min naps (quite ofter really) but this is usually when something is bothering her (teething, at the moment). Also, I think I was a bit slow on increasing her awake time as she got older - putting her down too early kept her naps at 45 mins because she wasn't tired enough so (want to) stay asleep after 45 mins. I think I was misreading bored signs for tired signs.