Thanks @defragster for the previous post link.
As I read the post, it leaves one with the impression that running at 600 MHz limits the Power On Hours to about 3 years. That is only the case if you are have a junction temperature of 95 degrees C and have it on continuously for the 3 years. Again, looking at the curves closely, the situation improves dramatically as you approach a more typical junction temperature of say 75 - 80 degrees C. It approaches 9-10 years of continuous Power On Hours.
My personal takeaway is that you need not shy away from operating at 600 MHz in most applications. Of course as stated in the post, using tempmonGetTemp() to find your actual junction temperature is wise advice.