Snare side you mean? 
The general rule i have heard is that as you tune and retune, as the head loses life, it will be harder to get it tightened up and stay that way nicely. 
With that logic the only way to tell would be to tune often and get a feel for when the thing has lost its ability to hold the tuning. 
That is from someone with very little drum tuning talent, so take it with a grain of salt.
Daav