At what ever level, productive practice is what counts. You can muttle around for hours on end and gain absolutely nothing. You need to slow down to correct mistakes, sometimes painstakingly slow to say, accomplish fingering or timing etc. So what ever that takes, then move on. As you progress it takes more time to get through more dificult parts because the more difficult parts are more difficult than in the past. But sometimes you can make progress in 30 minutes of direct assault on a certain run or cord arrangement. If that makes any sense !
I like to work till my head kind of fogs up on me, not so alert to mistakes, take notice. Then it's time to at least walk away for a bit, come back later. Yesterday that was about 3 hours of decent work. Went to the store came back, got another hour in, then went off to the mothers day events feeling pretty good about my morning session. Today I haven't started, heads already foggy. I might just do 30 minutes in a little while here. Just came in from cutting the grass, need a nice drink of water.
David