Depends on the genre. I'm currently playing Tales of Vesperia and it finally got interesting about 15 hours into it. I give RPGs longer timeframes to entertain me.
I put in about 25 hrs in Blue Dragon before I came to the conclusion it wasn't going to get any better and stopped playing it.
Devil May Cry 4 I gave 3 levels and were bored with all 3 and never played it again.
360 Games Now Playing: 360 Games unopened: Resonance of Fate Last 360 Game I Beat: Resident Evil 5
DS Games Now Playing: Dragon Quest VI DS Games unopened: Knights in the Nightmare, Etrian Odyssey III, Okamiden, Dragon Quest IX Last DS Game I beat: Radiant Historia
Wii Games Now Playing: Super Mario Galaxy 2, Arc Rise Fantasia Wii Games unopened Little King's Story, Sonic Colors, Silent Hill Shattered Memories Last Wii Game I beat: Sin & Punishment 2
