Romana proved that Timelords can program their regeneration if they're not in a rush...
Maybe there's a limited amount of emergency default regenerations if you are in a rush?
Considering that they can age backwards as easily as forwards, according to River in let's kill Hitler, it's possible that one "body" can last for millions of years... But then cell fatigue might be independent of outward appearance that if you keep the appearance of a 32 year old for 10,000 years, your body is going to get bored
... Wait! DS9 In The Cards, the crazy scientist experimenting with immortality by keeping cells excited, that's the police Lieutenant in Continuum... Fuck me, he lost some weight.