I think at least for me I'd change a snare side head every time I change the batter... snare resos are different than tom ones, they're much thinner, and if you're like me, I crank the absolute hell out of mine.
The last time I replaced one, I don't tension my snares very tight, but there were obvious marks from the snares all along the center of the head once I had de-tensioned it... definitely not good for the head.
If you want to be safe, I'd just change it next time you replace your snare batter.
When it stops tuning up and sounding good. I crank my snare reso very high, so I usually replace it when it's so stretched out that I can't get the tension that I normally desire. I usually change the batter and reso at the same time.