Originally Posted by doctor blue box:
“I don't know if this is just rumour or legend, maybe those in the know could clarify, but I have heard before that their was a three year rule imposed in classic who after Tom Baker, to do with the fact of how the production crew got fed up with him acting like he couldn't be replaced.”
I don't think there has ever been a rule about how long an actor should stay in the role.
Both Hartnell and Troughton did about 3 years each, but Pertwee stayed for five seasons and Tom Baker did seven.
Davison stayed for three seasons after Troughton had advised him that three years was a good length of time and would help PD avoid being typecast. When it came to it though, Davison actually wished he'd stayed longer.
Colin Baker was pushed out but knowing how much he'd actually enjoyed being the Doctor it's very likely that he'd have stayed much longer if he'd been given the chance. And although McCoy did three years too, the series was put on hold at that point. A fourth season with McCoy was already in the planning stages, and he'd shown no sign of wanting to move on, so it's quite likely he'd have stayed longer too.
So there's no sign of any production rule in place limiting the run of any of the classic Doctors. Just like there's no sign of any restrictions on 21st Century Doctors either.