For what it's worth, I've been doing my PPL(H) over this year, started in Jan. I might attempt my flight test next month (assuming I can get sloping ground landings sorted out). I've flown approximately once a week, for roughly an hour each (which was all I could afford). I have absolutely no prior flying experience whatsoever, and the groundschool has been completely self-study at a lazy pace - if you're determined, and have the time to spend, you could easily get through all your written exams in 4 weeks or so. Which is to say, I think I could.
Hours required that I've been quoted have mostly been 50 - 60 for no prior experience. As Whirls pointed out, though, having already attained a PPL(A), you should already have a good idea of things like navigation and radio usage. I believe you can also carry some ground credits across (I doubt you'd need to redo the radio stuff, for example), and I seem to remember you get a 6 hour reduction in minimums - obviously, this is all in LASORS.