I haven't been able to find anything that says exactly how long the THC from marijuana stays in mother's milk, but since it stays in the body for several days and can be detected even up to a month, I suspect it stays in the milk for quite a while. However, if this was a one-time thing, it's probably ok to go ahead and breastfeed as there is no indication that one-time use will hurt the baby.
Chronic use, however, is a different story. With chronic use, THC can accumulate in breast milk to high concentrations and theoretically affect brain development. A 1990 study showed that exposure to THC in the first month of life could result in decreased motor development by 12 months of age.
Chronic marijuana use can also reduce the amount of breast milk available. If you use marijuana on a regular basis, you'll need to weigh the benefits of breastfeeding against the risks of exposing the baby to THC.