I found that when I was learning, flying once a week or once a fortnight weather dependant, was a comfortable rate for me. Less immediate pressure to absorb the instruction and a chance for my bank balance to recover slightly as I block paid every few months.
The full thing (the old UK PPL) took about 18 months and as I was working durning the week and flying at the weekend I was able to maintain an income while I was learning. You may well be able to take a large chunk of time off whatever job you have at the time and do it in one hit, but who knows what you'll be doing a few years down the line? I would say start as soon as practical - should you get round to going after a job flying, my guess would be someone who's been flying for the past 3 or 4 years might appear bore attractive than someone with about the same hours logged who's been flying for the past 6 months. But as I'm nowt but a mere PPL it would only be a guess.