repot when you can see roots coming out of the bottom hole in the pot, as they are becomming rootbound. You can also tell by the plants behavior, color etc if it needs to repotted. my plants are big and had a long veg period so I repotted to 5 gallon buckets for the rest of the grow. remember that when you put the plant into flowering it will double or triple in size from the time you set it into flowering mode... figure out what your grow space is limited to and decide your veg time that way... Good luck