I think it would depend on the intensity that you set the M1 to and also the quality of the battery. I have two batteries that came with my M1 and another two that I bought from Amazon, and they all appear to have different charge life. Essentially, these made in China products are very crappy with no standard so they all pretty much depend on the luck you draw from getting good ones or bad. I’ve got one that will last a little over 4 hours when the M1 is set at an intensity of 10 and another that lasts only about two. The stronger the intensity, the fewer hours you can draw out of these batteries. But Bob suggests that you want to change batteries before they run out, as the charge becomes weaker with use.
I try to keep the M1 plugged into the socket rather than rely on batteries. I only use the batteries if I have to go out (very minimally) or if I have to make dinner, answer the doorbell, basically when I have to be mobile. But essentially, as soon as I can sit down or remain stationery, I don’t wait for the batteries to run down, I put them back on the recharger again.
I also wear a little apron with large pockets that I store the M1s (I have two) in when I’m moving around. And I always keep the extra batteries in the pockets in case I have to change them on the move. I don’t know how long these batteries will last, but if you need to use them often, I would suggest getting extras as back up.