As soon as your sharpener tells you that s/he can't sharpen anymore because nothing is left over. BTW my last pair of blades lasted for more than 15 years (with 3-4 times practice a week) and they weren't done by that time. I simply wanted to try another blade. Our local sharpener is a guru and well known for his accurate sharpening, just taking as little as possible off the material.
I think it also depends on how well you take care of the blades which determines how often you need to sharpen them. If you don't take good care of them, then you will need to sharpen them a lot more frequently.
A bad skate sharpener can quickly ruin a good pair of blade.
It also depends on the quality of ice you skate on. For crappy to moderately OK ice, they will typically last 2-3 seasons before they are too flat or there's nothing left.