I clicked on the thread by reflex when I saw "Sakura Taisen" and then remembered that this game is in English now too. How bad is the time investment, though? Is it about average for mobile games where you blow a bunch of stamina for about a total of one hour a day in a few bursts?
That's a little tough to answer. While early game may seem like that once you get a few ranks, AP restoration items are farmable. This means you can potentially grind once you have a decent stockpile of 1/2 Elixirs, recommended for some events especially Guild Wars. Add to the fact that you can restore AP and BP once per day by advertising to Twitter, you tend to not really be hard up for AP and start pacing yourself more.
AP restores at 1 per 5 minutes. BP/EP restores at 1 per 20 min and is used to join in other people's raids.
AP aside, there is a reason why some people call this Grindblue Fantasy. It's mainly because you're grinding towards endgame builds like you would in any MMO.
Welcome to Grindblue Fantasy. Enjoy your stay.