I have found that 15-25 minutes of practicing a study or technique should be followed by a 5-10 minute break, then do a different study in a similar way. The time goes by more quickly, and in two hours one can work on 6 to 8 different techniques or pieces. For technical exercises, breaking those larger time sections into even smaller ones is also effective: for instance, with a particular scale or arpeggio pattern, start quite slowly and work for 2 minutes, then move the metronome speed up slightly and repeat two minutes, maybe 5 to 7 times before taking a break and changing the exercise. This is the "salami" technique, where we find that smaller, bite-sized pieces are easier to digest than eating the whole thing in one sitting. Again, the time flies by when doing this, and one can be greatly assisted by having a metronome program with a timer attached, as I have in my phone. I also start at a particular tempo, say, 45 bpm, moving up 5 clicks every 2 minutes, ending at, say, 70 bpm, for 2 or 3 days in a row, then I push the start tempo up to 50 and the end to 75, etc. Seems very effective, because it's really easier to maintain focus for 2 minutes than for 10.
Lester Devoe Flamenco Negra
Lester Devoe Flamenco Blanca
Aparicio Flamenco Blanca with RMC pickup
Bartolex 7-string with RMC pickup
Giannini 7-string with Shadow pickup
Sal Pace 7-string archtop