I think some schools of thought would say to practice less but with more quality to your practice. But I like you like to practice for longer periods, as long as you can stay focused and are getting something out of it I think it can be okay.
In order to accomplish that, I try to mix it up. I have a large book of drills, and I'll vary it up and focus on some for a while then move onto others. Mix it up between shotmaking drills, patterns drills, focus drills, etc. I also jot down (or remember) any specific shot angles or position routes I've been feeling uncomfortable with, or missing more often than I should, and I work on those during practice as well.
I also spend a fair amount of time on I guess what would be called "practice playing". For 9/10 ball, depending on skill level throwing 4 - 6 balls out on the table and trying to run them perfectly (no hard shots, banks, etc.). If you get out of line, go back a shot and figure out what you did wrong. Do it 20 times, see how many out of 20 you are successful - again, remember it has to be clean. 15 or 16 out of 20 consistently and add another ball. It's harder than you think, 6 is pretty sporty, take good patterns, consistency, and focus. Also just playing the ghost to work on breaking and pure offense. And I might play a rack or two of just safeties, trying to hook myself and kick out of it over and over.
By then, I'm looking to play someone and kick their butt...
Scott