"Probably" on Showtime means that it could go on for years
-- as long as they can afford Michael C. Hall and the ratings hold, I suspect.
Regardless, I stopped watching after the dismal fifth season and don't see any reason to come back now. Even if season eight is the final year, that means the endgame will be drawn out over two entire seasons. I'll catch it on DVD if it turns out that worked, but I doubt it will.