Eventually sand becomes saturated with pulverized poop, you can't sift it all out, and when damp it will stink.
As with anything, it depends. It depends on how big the run is, how many chickens are using it, and how frequently you scoop the poop.
We have a covered run, ten years old, 128 square feet for eight bantam hens. I scoop poop every night for five minutes. The run still looks as about as pristine as the day we first put the sand in. No smell, of course.
We love sand. Wouldn't use anything else.