"larry moe 'n curly" wrote in message
...
When run at its rated power?

I'm referring to 1/4 - 10 watt resistors in TVs and PC power supplies,
probably in 30-50 Celcius surrounding air.

I thought that resistors were supposed to be chosen for twice the
actual power, but I've seen many 0.5W to 1.0W resistors run at almost
exactly their rated power continuously.

Serious question?

It depends on what it is made of.
Also resistors are measured in ohms not watts.
Genereally speaking the 1/2 life of the material is a good clue
as to when it#s resistance will have doubled.
SO don't use anything to radioactive.
A typical value would be a million triilion years, if it
fails before them take it back to the shop :O)