it depends on how bad you burned them actually. severe burns can take up to two weeks. you should know that affected leaves won't heal, but new growth will not be affected.
Well I burned the shit out of my leaves when the plant was only 11 days old, I changed the soil 3 days later (I would've flushed it but it was merical grow 3 month fert.) And now the plant is about 17 days old and the leaves are actually healing, and well.