Sean has something like 75K on his and it doesn't sound like he has replaced it yet. I've got 53K on mine and it seems fine. I think it should be good for 100K minimum.
I think the reason you don't get a lot of solid answers is that vitrually no one has replaced the clutch do to normal wear.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R