Really depends on the time of day. Istanbul's traffic most of the time is truly appalling. So, if you are travelling back from the Old City to the airport at peak hour or thereabouts, you may need to allow a couple of hours just for that trip! Same inbound. Seriously with 10 hours to spare, it could be tight. You'd need 4 hours to do the Blue Mosque, Hagia Sofia and (say) the Cistern (they are all within easy walking distance of each other), but if you wanted to do Topkapi Palace or the Spice Market or Grand Bazaar as well...your time is very tight.