One, as Nancy mentioned, use quality headphones. by 'quality' I am referring to those that are made expressly for use with metal detectors, have a good fit and use high-impedance speakers. Like Nancy (IL), I also prefer the SunRay Pro Gold or the similar Killer B 'Wasp' headphones. Not only will using good headphones help extend the battery life over simple speaker operation, but you'll just hear the target response better, and deeper.
Second, make sure you use a good alkaline battery. Rechargeable and standard carbon zinc or heavy duty batteries will not last as long.
Also note that if you plan on a doing day of detecting it will put more drain on your batteries, but if you only hunt an hour or two one day and then do it again several days later, the batteries wioll do a little self-restore that can extend the life a little.
Referring to the MXT Owner's Manual, it states:
Alkalines are recommended for use with this model.
During normal searching conditions you can expect
about 40 hours of hunting time from a quality set of
eight alkaline batteries.
Once the batteries become weak (8 volts) "Lo Bat"
will automatically appear on the display during
searching. At that point the batteries should be
replaced. Alkalines provide some reserve time
after "Lo Bat" appears, rechargeable do not.
I am usually alert to my battery reading getting below '9' volts and when it gets to '8' I just monitor it based upon where I am hunting and how long I might keep hunting. I have hunted for a good length of time after it hit '8' volts and dropped below that, but I was using top quality alkaline batteries and Killer B Wasp headphones. Swapping batteries at '8' volts might be your choice, as suggested in the manual.
Monte