What is your typical idle temp?
If applied correctly, one application should last for years. I think 3 years has been stated here before, but I doubt it's an exact science. I ran SETI for years here myself and it hasn't yet lost its potency.
If there are concerns about heat, make sure the heatsink is on correctly and that there is good airflow... more fans are good, but sometimes the intake vents get dusty. Room temperature is also a factor, as well as the case's own cooling potential... The Sonata is "adequate" with cooling, but doesn't win any awards.