When I have farmed things like wrath instances or BC instances on level 85 dks they lasted only around 3 days, but when I have farmed cata instances that are around the same level (lost city, grim batol, halls of orig) they last significantly longer. I'm currently at over 7 days /played level 85 farming.
Don't know whether it's a kills/hour thing, less player reports, queued for a ban wave, etc, but it's working for me so I'm sticking to it
I put them on 24/7 but with no relogger, so sometimes if they DC they get a few hours break lol