When you first obtain a copy would be how I'd phrase it, or when a copy is first attached to an account. I think that'd require Unity to actually talk to each shop their games are available at to know for sure when it happens, or maybe not.

Also, that'd open a loophole for games that don't require an account anywhere to download and play, or even games that you can play straight from the browser, Friday Night Funkin' being a prime example of those situations. Also, you -can- download and install a bunch of games from the Google Play Store without having an account, so that'd be another way to skirt that account check.