The one where you get 25-50 mythril on special events takes 1 battle - which takes a few seconds because you can one shot the enemy even at a low level.
Regular battles with 1 fight takes me 15-20 seconds (unless its a boss battle which could take me a min or two).
Normal battles (that get you 1 mythril) have 3 fights which usually takes me a min or two.
All of my characters are lv 99 (which is easy to get due to experience eggs you get from special events).
FFRK is really easy and not that grindy.
I find it a nice time sink when I have nothing else to do when I am outside of my home.
Like I said, your results may vary but I built up my party and cast of characters without spending a single penny on the game.
Well, I'd at least try it then. My current phone probably can't run it very well though.