Whatever suits you best, everyone has different views.
I intended to knock off a few hours while studying but in the end it didn't happen. There were other things away from aviation I had to catch up on. Once the exams were over I went straight for it in 6 weeks and had a great time. Very intensive flying really sharpened up the whole affair, and being involved in the flying club scene was great fun. I would recommend everyone to get away from people just doing it as a career step and spend time with guys doing it for fun. It is a real eye opener how much enthusiasm they have and they always have some pearl of wisdom to pass on.
Good points above, P/UT time is wasted when hour building. Don't just go at it sporadically.
Cheers
SK