mills grenade is 5 seconds tested it, m24 stick grenade is 5 sec, and mk2 grenade is 5 so they all seem fine (testing was done with timer and letting blow up)
Frag fuses in WWII generally did not exceed 5 seconds, and the fuse for F-1 grenades was even less than 4 seconds.
I hardly use frags but satchel charges in gams so idk how frags behave in game.