In reply to chris wyatt:
my empirical experience is that i get the biggest improvements when i manage to do a little, very often.
the ideal best would be to train 2 or 3 consecutive days then take a rest day, adjusting the volume and intensity of the sessions so that my body can manage it.
this has actually happened on a few climbing trips (3 climbing days 1 off, 5 or 6 pitches a day. " days on 1 off for bouldering and very fingery spots) and on a few months when i replaced the twice-a-week climbing gym session with a quick evening session at the nearest crag, whenever my fingers weren't sore from the previous day and i actually managed to find someone there to belay me or fix a line for me (i would self belay then).
The problem, is that it is very difficult to dose indoor sessions so that you can manage 2 or even 3 in a row without getting injuried, and i suspect that outdoor sessions are much more effective on the technical side.
the levels at which i experimented the above range between 6bOS/7aRP and 7aOS/7c+RP/7b bouldering. I can't say if it still works in the 8's!!!
another thing that i have reason to believe is that the higher the established level you have, the bigger volume and intensity you need to get past it.
And in reverse, the lower your establlished level, the lower volume/intensity you can afford without injuries.
twice a week and one weekend day can take you from 6 territory to the 7's quite fast, but won't make your first 8a unless you are very "naturally" talented.
2-3 times a week plus saturday AND sunday outdoors will probably get you in the 8a zone but might result in an injury if you are still in the 6's.
WC climbers train as much as 6 days a week, morning and afternoon sessions, try that being an average 8a redpointer and you'll break something.
etc...