I did between 2 and 3 hours per day. Any more I thought was rushing things, and I was starting to feel a bit drained. The fresher you are the more information you can assimilate, so if you're at all tired you're not getting the best out of it. Little and often is the best way, I feel. It gives you more time to reflect on each flight, mull things over and subconsciously continue your learning, then you're back in the seat again to consolidate and move on straight away.
I was doing this semi-intensively, 2 per day for 2 weeks inc. weekends. Factoring in weather I did 20hours. An unavoidable break meant no more flying for a few months, but as I went back to the same regime it didn't take long to pick up and I had no more breaks til I was waiting for test, which was preceeded by the mandatory pre-test check flight anyway.