Transferring isn't going to be too hard, but you might wanna look into getting rockwool cubes. That way you can just put the cube in the ground, no real problems and little root shock, if any. If you used the cubes, you're gonna wanna transfer when you see the roots starting to pop out of the sides.

If you don't wanna buy the cubes, or if you wanna give the ones you're growing now a fighting chance, it's gonna be a little harder. I'd say transfer at two or three weeks, a few inches high. Too early and they won't take, too late and they'll suffer a bigger root shock. When you're ready, dig a hole about the size of the pot. Next, gently tap the pot. Your goal is to try and get all of the soil to come out in a solid piece. Another option is to cut the can vertically, keeping the whole thing still one piece, put it in the ground, then try to take out the can.

Remember the perlite, still. You want your babies to breathe.