All soap should be allowed to cure for at least four weeks, for improved quality. However, homemade soap is usually fully saponified and safe to use after 48-72 hours. After two or three days, there is usually no active lye remaining in soap. There are some exceptions to this. Some essential oils, such as Lavender 40/42 can slow saponification, so that active lye may remain in the soap for up to a week. On the other hand, soaps made with Lavender 40/42 may not always be slow to saponify, depending on the process used to make the soap.
You can safely use your soap after 2-3 days. But the 4-6 week curing process makes for higher quality, longer-lasting soap. The curing process of soap involves a change in the crystalline structure of the soap. Soap quality continues to improve for a year or more after it is made, so in some sense the suggested 4-6 week curing time is arbitrary. But it is generally agreed that it takes 4-6 weeks of curing time for the soap to reach a generally accepted level of quality.