I think what Mike is really trying to find out is how long he can fly before his receiver battery is too low.

Mike, there's a couple of ways to find that out. Steve gave you one where you can work things out mathematically. To do that, you need to know the capacity of the cells. Usually that's printed on the pack. Even if you find that out, you don't really know their true capacity. Age and how you use/maintain them effects how much energy they can hold. The way to find out the true capacity is to use a cycler that will charge and then discharge the pack, keeping track of how much energy it stores. Maybe somebody at your field or hobby shop could help with that?

A more practical way in the field is to use an expanded scale voltmeter, or ESV. This tells you what the voltage is on your pack, and knowing that you can make a decision on when to stop flying. Some companies now make a little device you can install right on your plane to give you an idea of if it's safe to fly or not. Great Planes makes one called Voltwatch.

If your pack is relatively new and you've taken proper care of it, you can probably quite a bit of flight time out of your glider. Two micro-servos and a receiver shouldn't draw all that much juice.