Everybody's different, but if you are exclusively breastfeeding (not supplementing with solids, formula, or anything else), chances are good that your periods won't return for up to six months. They often return when you start introducing solids, but Aunt Flo might even hold off for as long as you continue to breastfeed frequently — as long as two years or more for some women. (Do take precautions in the bedroom though, unless you're ready for baby number two. A few women begin menstruating as soon as four weeks after childbirth, despite breastfeeding, and you may be able to get pregnant before you see evidence of your period.)
The return of fertility is delayed best by feeding baby frequently with no supplementation. As soon as baby begins to sleep longer during the night or starts receiving bottles of supplement or solid foods, fertility can return fairly quickly.