Wow, 7 hours for lithiums is very low. I used to have a beyer mv100 and would run 9v lithiums in it and got about 25 hours. I ran regular 9v alkalines in it a couple times and got 6-7 hours. I seem to recall that the beyer uses slightly more current than the 'sax, so I'd imagine you'd get at least 25 or maybe 30 hours.
Also, I once kept running the beyer on lithiums once the blinking low battery light came on to see how long it would go (for an amfibian show--no offense to phish and tom marshall fans, but that band just sucked). It ran for over 2 hours after the low battery indicator came on. After taking the tape home to listen though, it started distorting at about 1:30 or 1:45 due to the phantom power not getting enough juice I'd imagine. Anyway, bottom line, if you want to be daring, you can run it until the low battery light comes on and then keep going and change it at the end of the set and you'll still probably be fine.