At the very latest it should be available once you have “cleared” the launch patch. I don’t necessarily think as soon as you hit max level, but I do think the moment your main has done pretty much all of the main exploration stuff, all your characters should have flying.
By that I mean once you’ve finished the leveling campaign (not the time gated covenant campaigns, but the main storylines of each zone), have the entire map explored for each of the zones, and MAYBE collected like… 50% of the treasures, or at least found certain set treasures that required jumping puzzles to reach or something. Don’t even reputation gate it. Just reward exploration. The devs claim the reason why they don’t like flying is that we ignore all the world design, right? Then there should be no problem unlocking flying once we’ve seen and experienced that world design. For the lazy there’s always getting one of their more industrious friends to pull out a 2 seater mount, after all.
But under no circumstances should we be forced to wait upwards of 2 patches for Blizz to arbitrarily flick the switch and make it unlockable. I don’t mind the pathfinder system, even with reputation gating, but I do not think it’s ok to make us wait for a later patch to finish it. It should be launch content.
Oh, and of course, once a new expansion is released, just flat out unlock flying for everyone in the previous expansion. I know some people who still don’t have WoD or Legion flying and that sounds incredibly painful. If it’s not current expansion, unlock flying. Simple as that.