Currently i'm doing upto 4 hours flying a day, 5 days a week, using the off days and weather abandoned sessions to sit the exams. The weather has been a real hassle recently, however things are progressing and hopefully will have the PPL completed shortly. I bought a pack with all the books and a question/answer CD-Rom. Found it quite easy to read the book making notes here and there, then just run through the trial exams for a couple of hours.
4 hours a day is a little bit tiring, but definately not too much.
In hindsight I think I would have considered going to the USA for the good weather, but still i'm enjoying myself.