In reply to DundeeDave:
I've had two periods in my life of regularly doing short training sessions.
One was when I was commuting between Manchester and Liverpool, and used to stop at Pex Hill en route on nice mornings. Obviously real climbing is better than a climbing wall, but I fell into the bad habit of doing pretty much the same circuit every time on problems that I had totally wired, so probably not the most effective use of my training time.
The second was when I recently worked for a company that had a little bouldering grotto in the corner of the office gym. I had two 45 minute lunchtime routines that I used to do.
(1) Endurance. 10 to 15 minutes ARC circuits, 10 to 15 minutes playing on project problems as active rest / to relieve the boredom of endurance circuits on a small wall, another 10 to 15 minutes ARC circuits.
(2) Power: warm up with a short ARC session or technique exercises on easy problems, then pulling hard on project problems. Need to be very careful about adequate warming up and rest when trying to train power in a short session - not the most tendon-friendly activity.
Definitely felt benefits from these sessions, as they enabled me to train a lot more regularly than my one or two evening/weekend sessions per week away from family would otherwise have allowed. But you need to be very careful with shoulders, elbows, finger tendons if you suddenly increase your training load. Training in a gym was a plus for me in this respect, because I could throw in a few sets of rotator cuff & wrist extensor exercises as part of my warm down.