I recall reading about a music study that determined 20 minutes is the efficient limit for the brain during practice.
I break my 20 minutes practice down with fretless or upright intonation, scales (!), bowing technique, reading and actually playing tunes or pieces (classical). Each 20 minute segment is timed and I take a break between them.
Works out well as I enjoy each type of practice segment. I mix up the sequence to what suits me and if I feel like a longer segment or eliminating something it's no sweat.
Yesterday instead of organized practice I grabbed the fretless Alembic Essence, grabbed a cold beer, cranked up the Bassmaster xs800/Powerhouse 410 and fired away at volume!