Well normally at 10,000 feet AGL. The reason being lots of things start to happen then like lights seatbelts and flaps you start to slow. But some pilots might want to do more flying and some even turn of auto pilot at cruise as they are bored.
I have only turned on autopilot once in a 172 flying from John Wayne to Fullerton. I did not like it so I turned it off and flew my whole decent and cruise I was only 1500 feet high though. I also flew right over Disneyland on that flight!