Veins heel very quickly; I have 4 injection sites I rotate around, and by "site" I mean a 6-inch strip of vein in my left and right arm (that's 2 sites) and the tops of both hands (sites 3 and 4).
The hands offer at least 3 or 4 veins each, so when I hit that "site" naturally I switch which vein I'm using, as with the location of where I hit that 6 inch vein in my arms. So, since I inject 4 times a day, I give each site only 24 hours of rest in between hits, but the exact location I inject (which I think is the important part) probably gets at least a few days before i come back to it.
I'm guessing it's the same with most of you? I avoid injecting the exact same spot too often, but I come back to the general area approx every 24 hours, which seems to be working out fine. My veins remian healthy, although the track marks in those areas basically never go away - more than anything, I'd say THAT'S the primary reason not to come back to a site too often, but it's hard to avoid; if I wanted to keep all visible signs hidden, I'd have to give each site about a week off in between hits, which just aint gonna happen, unless I can come up with 7 injection sites, and given how hard it is to find my veins at all, I just don't see it happening.
Anyway, I guess my overall point here is I think 24-48 hours is probably enough time for a vein to heal before you come back to it, assuming you are using good injection techniques (fresh, thin-as-possible needles (try to keep it to 27 or 28g MAX - I'm a big guy and use 29g all the time with no issues, so you can too). Don't use the same needle more than twice (hey in a perfect world we use a fresh needle every single time but I bet most of you don't - but you really should try to keep it to every other time), keep everything clean, put pressure on the injection site after you pull out and my personal favorite tip is to run warm water over every injection site after i hit, even if i don't miss (i find it helps healing and minimizes post-injection bumps and mosquito bites).
I also rotate skin creams and try to apply vitamin E, neosporin and Biafine (an excellent healing agent) after every session. Regular application of moisturizer and the ocassional exfoliant of injection sites is also a good idea.
Sound complicated? It really isn't once you get into the routine and, after all, if you're going to choose to be an IV drug user, practicing some healthy skin habits is a good idea (also, l-lysine as a daily pill supplement is a good idea, it helps aid healing skin).