I have plenty of students that show up and have the same standard receiver battery that you do. A good rule of thumb would be 2-3 10-14 minute flights. Anymore than that and you'll be pushing your luck. As said above, the best money that can be spent right now would be to purchase an Extended Scale Voltmeter. It's important to get an Extended Scale meter over a standard voltmeter because it gives you a better picture of the condition of your battery. The ESV measures your batteries with a load on them which will approximate the load that the radio equipment puts on the batteries, so it's really giving you a reading of what the battery has available for when the radio is in use. In addition, there are some problems (such as a dead cell) that won't show up until a load is placed on the battery. A regular voltmeter won't show you these problems.

Hope this helps

Ken