My recommendation would depend on whether you are aiming for one or several hits. If one hit is all you need then keep it in as long as possible, if you are taking several hits there might be a point when it's more efficient to breath out and take the next hit.

Usually when I exhale I see very thin vapor but I'd rather exhale slightly early than almost pass out from trying to keep in the smoke.

In my experience the key to a pleasent journey is to be relaxed. When you are desperately trying to get as much for as long as possible you'll stress your body too much to enjoy the trip.