Actually depends on ur age...
if u r below 18, usually the power will change about 50 to 150 every year. That's a rough estimation. That's the reason why Lasik/ lasek is only offered to ppl at least 18 years or older. some actually prefer u to be at least 21. that's when ur "eye power" becomes stable.

and of course when ur specs starts to show all the scratches, it's time to change..