You can rebreed right away after she loses the litter.
as far as a rebreeding and weaning scedual goes, the best thing to do, and the most responsible IMHO is to rebreed her a couple of weeks after they are weaned (at about 6-8 weeks of age). That gives the doe time to recondition. Otherwise you are going to take chances of really stressing her out and wearing her down if she's carrying a litter while nursing the first one. You want to give her a few days to a two week break in between breedings and weanings. Making them care for two litters at a time can put a lot of stress on them and burn them easily.