I know this is a little late to reply, but just saw your post. I usually wait until the next day, but make sure the butterfly has a good place to "hang" so wings can dry straight and it doesn't fall. I have a larger container that I have put nylon window screening in that they like to hang on, but make sure they can't get caught behind it and flap until the wings tear. That didn't happen to me, but I have been worried and try to prevent it. If I see the weather forcast is bad for the next day, I wait probably at least 6 hrs before releasing, but on a sunny day I suppose it would be OK to put out sooner on a nectar source. Also, sometimes you can just tell they are flapping alot and ready to get out, in which case I let them unless I am saving them to release with a class. They usually go toward sunlight, so moving the container to a darker spot usually settles them down. The book "My monarch journal" says it's OK to release onto a branch after an hour or two, but it's wings will still be limp. I was always afraid it might fall and the wings would deform, so that is why I keep it a little longer and watch over it.