pulled this from another forum:
We talk about heat cycles as it effects the attributes of the tire's performance. A heat cycle is generally referring to a full heating (or overheating...) to operating temperature and this only makes a difference if you are running a tire that is soft enough that the tire will generate more grip when heated until it reaches a maximum temperature that when exceeded causes grip to fall off.
The reason tires tend to go away after a certain number of heat cycles is that the rubber vulcanizes more with each cycle. Vulcanizing is basically the process of forming additional crosslinks between molecules. Most of the things I have read say that vulcanization will continue for up to 24 hours after a heat cycle.
The degree of this change will be very variable depending on the exact formulation of the rubber, temperature that is achieved and to some extent the force that the rubber is subjected to during the heat cycle.
In other words: There is no one answer. Experience will teach what your car and your driving style will demand and what the results will be.
Some tires will cycle out very quickly. Often times these are the best race tires. This doesn't mean they become dangerous. It means they become uncompetitive. For example Hoosiers and to a lesser extent Kuhmo Victoracers are known to cycle out pretty quickly. Toyos are known to be almost cycle proof. In between lies a universe of tires.