Read your altimeter instructions for the correct battery information.
The Duracell 9V battery datasheet is here.
https://www1.duracell.com/oem/Pdf/new/MN1604_US_CT.pdf
Look carefully at the lifetime and current draw data. This is a pretty good indication of how long your battery will last. The information on the lower lefthand graph on page 2 give you an idea how low you an go with voltage, and the upper lefthand graph on page 2 give a rough idea of lifetime versus on current drain.
Both the MAWD and the Adept22 draw less than 10 ma and 100 mw. Their data sheets suggest that lifetimes are up to 48 hours. The MAWD uses a capacitive discharge pyrocircuit so it doesn't draw directly from the battery so it can probably go a bit longer than the Adept22 which draws the pyro current directly from the battery.
Older altimeters like the RDAS Classic use a lot more current and therefore drain the batteries much quicker. The RDAS draws 80 ma so the 1000 mw or 100 ma drain information should be looked at.
Measuring the battery voltage
without a load will not give great infomation. You really want to built a test rig that temporarily puts a load resistor in parallel with the battery using a momentary switch so you can test the battery voltage under load. A 1000 ohm resistor simulates the voltage in a MAWD or Adept22, a 100 ohm resistor simulates an RDAS Classic, and a 2-10 ohm resistor simulates an e-match. I wouldn't want an altimeter battery to drop below 8 volts in a circuit, or below 5 volts when firing an e-match (or prevent computer reboots).
Bob