You might be suprised to know that each and every cpu will have a different lifespan due to the actual build quality, as for the heat sensor i did look on the schematics and the data sheet for the arm cpu used in the N900 and came to the conclusion it is just too much effort to install an effective temp sensor so i went for the next best thing and a heat sink will keep down the temprature but to be effective there must be better ventilation on the N900 for heat dissapation to work effectively, so a lot of headaches lol.

I would not take any notice of apps made for the N900 as the main goal is to keep heat to a minimum but if you incorporate a fan then obviously a heat sensing circuit would be beneficial but not imperative.
Only problem with a fan is the battery life would drastically decrease unless on an external power supply.

After quite some exploration on different N900's my estimation would be something like 18 month at full overclock for the pcb to get brittle enough to start giving problems with no heat sink, having said that with a heat sink applied would make that a lot longer lifespan on trouble free operation.

No way will the arm cpu in the N900 break down under normal conditions but i cannot say the same for the pcb it is soldered too.

All in all i will give the device a max of probably 5 year or more under normal speed before pcb deteriation starts to be a problem.