Heroin/water solution will eventually deacetylate back into morphine. Or 6-mam. Although the degredation rates are not really clear. You're shot should be fine in a couple hours, although it isn't good to try and leave shots of dope sitting for more than that. Not only due to the degredation of the shot, but also unless your solution is totally sterile (micron filtered using bacteriostatic water) you also run the risk of bacteria growing.