I pick my basil when the leaves are about an inch or so long. Also, it's important to pinch off the tops of the basil plants so that the leaves all get bigger, rather than ending up with a bunch of tiny leaves.
As soon as you see some leaves! The basil will grow a few inches a day. I'd say once either is over 8" tall, go for it. Just leave about half the leaves on the plant.Don't wait too long on the basil, as the older leaves will get bigger and tougher the older they get. The oregano leaves stay about the same once formed.
After they are leaved out you can use any time, just try not to over pick it could dammage your plants I take a few off every two or thee days of my largest leaves and dry them, I love Basil & Sage never have grown Oregano, Good luck.
when the leaves are as big as you like. But herbs taste sooooo much better fresh so only pick and freeze enough in case they trail off during winter. Be sure not to take too much either, because over pruning can kill.