I think I'm doing both...long practices made up of short sessions.

I do a morning and an afternoon session. I consider them long...at least they're much longer than I ever thought I would practice or ever did when I was a kid (dropped the guitar in the early 70's, picked it back up 11 months ago). But those two long sessions are broken into at least three, and as many as five shorter parts; e.g., GT lessons, scales, chord changes/progressions, songs, finger exercises, etc. These parts of the long sessions are in turn divided into their own short segments; e.g., individual songs or different scales.

It all adds up to a lot of time but the time goes by quickly and there's no fatigue. Practice is varied and if one part is going badly or is boring or otherwise isn't much fun (rarely the last two) it's not long until I move on to something else.

BTW, the short segments have scheduled amounts of time and the timer I use doesn't start until fingers are on the strings. There is very, very little time wasted or frittered away. I built up gradually; basically starting from scratch at 10 minutes a day. Now I often hit three and a half hours daily and the time is still increasing.

I think this is working very well for *me*. I never dread starting practice and once into it I have a ball. :)