Alright, so I got up to the point of return for the bulk of timed quests. Not all of them, some of them expire in stages before here, so the general rule of thumb is when you're getting to fight level
enemies in dungeons, finish off any previous timed quests in fields/dungeons and clear every quest in your current dungeon before moving on. You don't need to worry about the towns at this point though.
After that (aka when you should be worrying about towns) they don't specifically mention that you're at a point of no return for quests, but you'll be asked "Are you ready? [extra text box here deleted for spoilers] Are you sure you're ready?" (paraphrased) followed by a yes or no question in order to proceed after a bunch of awesome cutscenes. Make sure to do
any and all timed quests you want to do that you've got left at this point.
For anyone concerned about Heart-to-Hearts, apparently none of those are missable. Too bad I found this out
after spending hours farming certain character affinities. Oh well, not like it was wasted time, I'd have to do it eventually anyway.