Well... at least four years with Blackcomb being released in 2010. Service Packs inbetween to fill in the release gaps. I think it will work; personally, from what I've seen of Longhorn it will keep the people who are teetering on the edge from dropping into the Abyss (represented by Emperor Tux and his band of devilish Balrogs of GNU). Possibly moving some old XP users back onto Longhorn. The others... they're unsaveable as they moved to Linux merely because they dislike Microsoft.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.