you can use it at two weeks but it won't last as long in the shower as one that cured for 6 weeks. the cure time is the necessary time needed to allow the excess water to evaporate and give you a hard bar that will last. i have been known to use one a couple of days after slicing but it doesn't last as long as one that is older.
cure time varies by soaper, the method and the ingredients
some say 4 weeks, some 6 some 8 weeks.
hot process you can use right out of the mold, but it benifits to set for another 2 weeks.
soaps that are mostly olive oil tend to take abit longer to harden then a soap with a blend of different.
handmade soap is like women and fine wine it gets better with age.