Even 100%-ish is pretty crazy, like getting all the ff9 chocograph must takes roughly 10 hours, but that's necessary to get Zidane ultimate weapon. If you want the zodiac spear in 12, you might as well save yourself time and just go insane now.
I think if he wants to keep post coming semi regularly and not burn out hardcore it makes more sense to just do side quest that are important and have reasonable amount of content (say getting anima in FF10) but skip all the ultimate weapon and getting all the reward from blitzball. FF10 rushing trough content (no cutscene skip) is probably 30-40 hours, doing all the major side quest (getting anima, clearing that stupid dark dungeon) probably add another 15. But then doing all the really hardcore sidequest (completing monster arena, getting all the ultimate weapon) probably add another 50 hours, and then doing the really crazy hardcore stuff (maxing out stats) must add another 50 on top (there are trick to get a shiton of exp very quickly, but replacing all the grid with +4 stats require grinding like crazy for those).
Roughly on top of my head *(Basic clear, +most side quest, +absolute full clear):
4: 25, 40, 50
5: 25, 40, 50
6: 25, 60, 90
7: 35, 70, 90 (I got my save file to 99:59:59 trying to max out Cloud stats, but I was a dumb kid)
8: 35, 60, 90
9: 35, 70, 80
10: 35, 85, 135
12: 30, 80, No
13: Why?, No, really why?, I'm just calling suicide hotline now
15: 20, 60, not sure