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1500 hours in D1/2 combined (Destiny), so you know what you're letting yourself in for potentially.
Oddly enough I bounced hard off DII because I spent so much time building a character I liked, only to find out that it would get insta-gibbed by almost anything in Torment difficulty. Just lost any real sense of momentum after that although I did dabble a bit in online. Was young enough that I just stopped playing rather than trying out a different build (and was before you got a re-spec item after clearing each difficulty)
DIII ~ 520 hours
I probably have twice that in DII but that was before anybody tracked that.