When to put a red eared slider back in to a pond?

It really depends on why you took it out in the first place! When you say "back in to a pond" I assume you are referencing to having taken them out for a specific purpose or reason, in which case I would say put them back in when you are ready the issue has past. If you are talking about how long they can be out of water, I'd say max 4-6 hours.