When being alone I prefer to practice for about 1-2 hours. I' ve heard that your mind writes new information for a quarter and then it kinda repeats those for 3 quarters. This means that stopping every hour for 5 min. is essential. However it's extreamely important to practice EVERYDAY. Even 10 minutes. Practicing for about an hour a week by doing 10 min. of practice every day is much better than playing 6 hours in a row on Sundey and not having touched the instrument for the rest of the week.
Drummers like Dave Weckl have claimed that they practice like 12 or more hours every day. I belive him but very few people have the ability not to talk about the patience to do that. 1-2 hours EVERYDAY is the best IMO.