Honestly if I were to guess, I'd say around 800. 500 is too low tbh considering that pug.na =\= tf2mix, so the target skill-range is kinda different. Plus if a player has less than 800hrs, they're probably an alt so yeah. There is also the issue of a cs player coming in to play tf2, but I guess that can be solved by having some sort of verification in place like confirming with an esea account etc.
also charis, while this won't fix the issue with alts, it'll at least greatly reduce the number of them/discourage people from doing it