Star Wars Galaxies (pre WoW), about 12 months
WoW, about 9 months
Travian - browser based, does that qualify? about 6 months
Also a game I can't remember the name of. Was in beta for ages, and looked like an isometric RTS except you only did capture the flag type missions against other players in 8x8 or 16x16 battles. All I remember is people screaming 'poc leh'. about 6 months. (edit: it's called shattered galaxy and it's still running)