I used my Max2 as a monitor at least 10 hours a day, it lasted a year before battery went. But i had it plugged in all the time because I like to use the power hungry a2 mode, so it could no doubt last longer.
I don't know if it's possible (for me) to swap the battery's lead with that of another battery or if I would need to find a shop to do it for me, and could not find an original battery...and onyx wouldn't send one
But I Bought a spare Max2 just before it died so have been using that.
I suggest you get a spare battery if you can in advance.