This information was gathered for a question on the RAMPS newsgroup for a UM200 transmitter but should be proportionally the same for other transmitters.
For newer tests on the iPower LiPoly rechargeable 9 Volt See (FAQ: How do the rechargeable 9 Volt iPower batteries compare to other batteries? How long will they operate my unit?)
Here's some more battery information as I promised a few days ago. It took a while to run all the batteries down. Here is what we did: we used the same transmitter, a Lectro UM200 for all the testing. This is a 100 mW UHF belt pack transmitter. This particular unit pulled 75 mA. We ran four different kinds of batteries to a final voltage of both 7.0 and 6.6 Volts. 7.0 Volts is where the LED is pretty dim and where two of our receivers with battery readouts start indicating low battery and 6.6 Volts is the very low battery indication. The transmitter is getting close to completely dying at 6.6 Volts but will usually run to 6.4 Volts or less. The LED goes out totally at 6.8 Volts. I'd put all this in a table but I don't think it would survive the news readers' formatting. So I'll list the type of battery and then the very dim LED point (7.0 Volts) and then the maximum use (6.6 Volts). Your mileage may vary.
Here's my conclusions: Assuming that a sound mixer with good common sense would toss a battery when the LED is very dim (or sooner) and using an standard alkaline as a reference, you'll get 3 times the life with a lithium, about 40% more life with an Ultra alkaline and about one half the life with a top quality nickel-metal hydride (NiMh) battery. (Though the Varta NiMh claims only 150 mAh, they start out at more than 180 mAh.) Also, the NiMh batteries don't self discharge as quickly as NiCad batteries since the battery after sitting for two months was still at close to full capacity.
Disclaimer section: These were fresh, new batteries at room temperature. This was just one test, performed on just one transmitter.
Anti-disclaimer section: Most brands of alkaline batteries are about the same, alkalines and lithiums have a long shelf life, and our transmitters are pretty consistent. We have found the Eveready batteries to give the longest life for a standard alkaline battery. In any case, the ratios of battery life should be good numbers. You guys know what kind of battery life you are getting now, and the ratios should be informative.