Now that just depends on how you are shooting now. If you are shooting fingers and plan on changing to a release then about 3/4" to 1" shorter is the norm. However, being as most people shoot about 2" too long in the first place then read my next paragraph.

If you are just starting out then measure your wingspan and divide it by 2.5. This will get you closer than any other method. And get a bow that is adjustable till you have about 20 years of experience and know exactly what your draw length is.