Thats up to you cole, it depends on how big you want your plants to be. The rule of thumb is generally to let them veg until they are half or a little more than half the size you want them to be when they finish, before you switch to flower. That can vary depending on genetics, and how they were bred. I know at least a strain or 2 from "Mandala seeds" claims to go antywhere from double to triple the size when you switch to flower.
peace