There is no direct flight from Heathrow to Cincinnati so the travel time would depend on the time at the connecting airport.
8 or 9 hours
exactly 8 hours 42 minutes 11 seconds and 39 milliseconds on Jetstar. exactly 8 hours 23 minutes 15 seconds and 52 milliseconds on Webjet. exactly 9 hours 54 minutes 39 seconds and 22 milliseconds on...
12 hours
When we went it was just about 9 hours maybe 9 1/2 at most. The return flight was about an hour longer at least 10- 10 1/2 hours.