It varies, I shoot for a couple of hours at least, but often it's only an hour or so. What I work on, and to a certain degree how many hours, depends on what's going on or coming up. I might work on the toughest parts of the tunes I'm working on lately. I might work on moving chords through inversions up and down the neck, or playing arpeggios or little patterns or phrases, and moving them (usually) chromatically up and down all over the neck -- playing them forwards one pass, backwards another pass, then alternating forwards followed by backwards or vice versa. I might work on the scales that I'm currently trying to get a grip on and apply in my soloing -- specifically, these days, the Mixolydian mode, the minor pentatonic with a 6th instead of a dominant seventh tone, the diminished scale, and the jazz minor (ascending melodic minor starting on the seventh)... or lately I'm also trying to read and sight-sing intervals and stuff... then I'm trying to play chords and melody and walk bass lines and intersperse comping like Joe Pass or Tuck Andress... lot of stuff for an hour! As a result, I mix it all up so much that it takes me years to get anywhere. I need focus... or several more hours I can use for it.