Far, far too many.
I make new characters (and delete them) like crazy, but my most recent main is only at 98 days played. Made her during WoTLK.
On my main and I'd estimate an additional ~120-150 days across the rest of my characters.
Im too afraid to check, i know at one point in Cata i had altoholic running and it tallied all the toons on my server at something over 600. I imagine its around 750 by now