A tag actually can't even exist until there's a question on the subject, so in all likelihood, that means when playtest actually begins.
For reasons we discovered after the D&D 5e playtest, that tag should also be something separate from whatever tag the non-playtest published edition is eventually tagged with. (It turns out that too many questions on playtest material simply don't match the later game enough to share tags, since they're effectively different rulesets. And it's easier and less confusing to searchers to update still-relevant questions to the non-playtest tag as we find them, than the other way around.)
So in practice, it will have to be when the first question is asked, and the tag name should be something like pathfinder-v2-playtest or something like that (playtest game's working name +
-playtest), much like our dnd-next-playtest tag.