It makes me sad to even reply to this thread. We live in what should be the most cat-perfect area. A half mile from the nearest road with any traffic. Surrounded by woods.
We've now lost two of the three indoor-outdoor cats we've had while living in this house. One got run over at 5 pm on the road a half mile away. The other took off hunting one night and never came home, most likely gotten by a fox or a fisher cat. And, that doesn't count numerous bites and injuries over the years, obvious close calls after going missing for two days, and being tree'd at ridiculous heights when all you can do is not look and hope they figure out how to get down.
Our two new kitties will be the first indoor-only cats we've ever had. I just can't go through losing more pets and the odds are proving to be horrible. I'm not happy about taking away their freedom and instinctive hunting. I just can no longer convince myself that it's safe in the woods.
My vet did offer one alternative. Withhold dinner until fairly late at night, training them that dinner time is, say 10pm at night. Then, let them out for a run early evening, using the instinct for dinner to bring them in for the night. If I were going to go indoor-outdoor again, I would strongly consider this approach for risk minimalization.
To try to answer your original question, I think you have to play it by ear and judge when the cat is fully comfortable in his new surroundings -- eating, etc. Then maybe a short trip outside with you, right before supper time so you can judge how easily he returns for food.
Has this cat been indoor only or indoor/outdoor in the past?