What is the normal expected life on the gas rings?  

It really depends on cleaning and proper lubrication. Some will last longer than others.

Here is the test that will tell you if they need replacing short of a short stroking problem.

Take the bolt out of the carrier. Place the bolt back in the carrier less the cam pin and firing pin. Hold the carrier with the bolt facing down. If the bolt slides out of the carrier under its own weight, the rings need replacing. Replacing the gas rings does change your zeros a bit so that if you do it all the time, your POI will shift. Make sure you keep the rings lubed with a good oil or CLP, this will greatly extend the life of the rings.

The other test that I have usually used is to put the bolt in its extended position in the carrier and place it on a table. If the carrier drops on the bolt under its own weight, I replace the rings.

Very good then.  I though I was still OK but with just shy of 2000 round through my service rifle I compete with I thought I should ask.  On any of the other ones I wouldnt even worry about it until something went wrong at the range, but with this one I don't want to find out during a match!  

Thanks!