In the past I usually tag the animals when they are going to go into breeding, at the request of a new owner, or when we are about to go to show. So anywhere from 5 months to about a year.
They don't freak out when you tag them. You do it quickly in the right ear with the brand facing the inside of the chinchilla's ear. If you watch someone do it at a show, it takes a couple of seconds. Just grab the ear, tag and they're good to go! There are not many veins or pain receptors in the edge of the ear where we tag, so there is rarely any bleeding and most don't even feel the tag go in.