I practice in several bursts, that probably amount to about 3-4 hours a day. I wish it was more, as I'm a full-time musician, albeit not performing at the moment (working on an album). Last year I was practising a lot more each day but you go through phases, with different levels of motivation I guess.

With the guitar in particular, like people have said, smaller more regular spots of practice throughout the week are much better for retaining your muscle strength, timing, dexterity, etc. I think another key is to ALWAYS challenge yourself, if you have the time to do so and everything. I've spent the last few years periodically hunting down pieces that are that bit beyond my ability and learning them bit by bit.

Singing is another matter for me, though. Perhaps because I've just started; I've learnt a small handful of Bert Jansch songs and I'll sing fine one day and absolutely terrible the next. There seems to be no control or consistency - I hope that it'll get a bit better with experience as it's driving me to distraction!