There is no information on the AS website to indicate that you should reapply AS5 after a specific period of time. However, my personal experience and many user forums are all of the opinion that over a period of several months, AS5 dries out and loses thermal conductivity. Of course, depending on your specific situation (oc or not, high ambient temps, good/poor case air circulation, etc) it may take nine months or two years for AS5 to dry out. But, regardless of whether it is nine months or 2 years, the fact remains that AS5 will dry out and the cpu will realize increased temps.
A google search will return any number of forums and articles that are of the opinion that AS5 should be re-applied after several months of use.