I had a chat with the blood bike guys at the NEC bike show last year. I was told they like you to commit to two days a month but are flexible as to what days etc. It is on call so sometimes you do not do anything, but need to be available if you are needed. I was told it was mainly out of hours (ie evenings, weekends etc) when the other usual transport options are less able to do it. This is what the guy told me, it may vary perhaps a little group to group. I too was interested, I love riding and to help someone while doing so is even better. But for me with work and personal life, at present at least, I could not give them the commitment they need/deserve. Those that can achieve it around a day job etc have my respect.