I re-use color developer (Paterson Photocolor II) once -- that is, I put two 35mm (generally 36-exposure) rolls through 275 or 300ml of solution. The instructions for this developer indicate it can be re-used three or four times, IIRC, but I'm erring on the side of caution. Once diluted, it tends to go bad in a couple of weeks, so depending on quantities, film area, etc., you might want to save up your film and process a bunch of it at once. The Paterson developer keeps for a few months in its undiluted form, at least in full bottles. In partly-full bottles, of course, it goes bad faster. I'm thinking of getting an inexpensive vacuum pump (discussed briefly in another thread here on APUG recently) to help extend the life of developers in bottles. Sadly, the Paterson chemistry has been discontinued. I'll probably either switch to Kodak C-41 developer or use a mix-it-yourself variety. Incidentally, I've also tried the mix-it-yourself (there was a url link here which no longer exists) which is a divided developer for C-41 films with a claim of long shelf life. In my experience, it sometimes works well, but it often produces thin negatives and occasionally produces odd colors that I've not been able to correct in scanning or printing, so overall I don't recommend it. A pity, since the claim of long shelf life is a big draw.
FWIW, I once used Paterson's E-6 kit ("Chrome 6," I think it was called). I had little luck re-using the developer, though; I suspect it went bad in just a couple of days. I'm now using a Kodak E-6 kit one-shot. This is expensive compared to the Paterson kit, but it's been much more reliable, and it's still less expensive than sending my E-6 film out for processing, assuming the kit doesn't go bad before I'm finished with it. (I've been using this one kit for a few months now, and so far it's been OK. Obviously I don't shoot a lot of E-6 films.)