For what it's worth, I took around 55 hours from scratch, flying once a week, over slightly more than a year. This includes 3 aborted tests (for weather) which include pre-test polish. I've never flown a fixed wing in my life (they make me air-sick, sadly).
You should have some idea of how long you'll need by the time you hit 20 hours or so; if you're really struggling, you should probably budget a bit longer. Also, as has already been pointed out, the more training you can get into a shorter period of time, the less time you'll have to spend re-learning stuff. Flying about an hour every week, I was spending the first chunk of every lesson relearning what was covered last week. If you could fly, say 3 or 4 times a week, you will get through in lower hours - I only did the 1/week plan because that was the most I could afford. To this end, it's probably good idea to save up initially rather than rushing straight in and doing things in dribs and drabs, assuming you can wait that long
