I guess I'm in weening days of the "newbie phase". I started Starting Strength in June and have doing in since. My bench started slowing about a month ago... giving me about 5 weeks of growth. This wasn't unexpected, though, because prior to working out correctly (after finding T-Nation), I had just done dumbbell work and some bench for a couple months before.
Squats are still moving up, but not as quickly. Up to 260 for 3x5 now, but it's rough.
I'd say a three-month period is probably about right, at least for me. My bench added about 70lbs, my squat about 100lbs.
Likely it's relative to the muscle groups, too. I never worked legs initially (being the stereotypical ignorant teenager) and now that I've begun, strength is increasing rapidly in comparison to what it will once my body is accustomed to such.