Manufacturer's recommendation for running shoes is often to replace after 500 miles.
Bearing in mind I average 50+ miles a week a year, that would mean every ten weeks for me and I'm far too tight to be doing that.
I tend to replace shoes after the cushioning starts to go, which is about after 9 months.
Depends how injury prone you are. But I reckon if you are running <25 mi a week you can go well over a year.