My guess would be around six months. A lot is going to depend on the severity of the clip and the feather growth of that specific bird. Some birds will surprise you and be able to fly or at least glide in 3 months after a wing clip others may take up to a year. If you are wanting him to be able to fly just be patient, he will probably start doing a lot of trying to glide from cage to floor when they are beginning to get long enough, Just make sure he is not too high up when this happens.
If you are not wanting him to fly then check the wings at the beginning of each month to see how they are growing and when they start to become long enough for flight. Some birds suprising enough will fly even without the full growth of their wings. Many "lost" birds are ones that their owners thought their wings were too short to fly but they managed to fly away anyway.