I've sunk more hours into it than any competitive online shooter this gen. And for me, that's saying a lot(was a huge CoD fan pre-2010. Was a huge Halo fan until the turd that was Halo 4. Still play quite a lot of BF)
Last time I checked, I have around 300-400 hours of total playtime in BL2. And about half of that is in single player. It's only barely starting to feel a bit stale to me after a whopping dozen+ days of playtime.