I guess this will be my little compilation post, with some detailed repeats.
Splinter Cell: Conviction
Solo story (Realistic): ~8 hours
Co-op story (Realistic): ~6 hours
Hunter on all non-DLC maps (Realistic, solo): ~6 hours
Last Stand on all non-DLC maps (Realistic, solo, starting on wave 15): ~6 hours
Transformers: War for Cybertron
Full story (I-X, Easy), including all story-related achievements, destroying symbols along the way, playing the last checkpoints on Hard: ~12 hours
Escalation (wave 15, solo): ~3 hours (less than half that with a full team)
Ranking from 0-100 in solid boosting (no XP events, no DLC maps): ~32 hours
Yes, I played MP for fun for a while

. This is an estimate based on ~8 hours with one class.
TimeShift
Campaign (Elite): ~8 hours

XBLA:
Harm's Way
200G: ~20 minutes

XBLA:
DeathSpank
200G: ~8 hours according to Book-O-Stats

XBLA:
DeathSpank: Thongs of Virtue
215G: ~8.5 hours according to Book-O-Stats
Bayonetta
1000G (including halo farming millions for items): ~26 hours

(assuming the same for

)
Oddworld: Stranger's Wrath
Story: 11 hours
Portal 2
Story: 9 hours