With any tumor, if you feel that surgery would work, do it. If the vet feels that itis inoperatable, watch your mouse for signs that she isnt eating or drinking. If it looks like she isnt moving around as much, or looks sickly, and trust me you'll know, than it is time.
One of my rats that i recently put to sleep, I loved him a lot and he was a hairless rat, so his was really seeable and was on his stomach. Till the day he was put to sleep, he was eating, drinking, struggling to get around, but he was getting thinner and thinner. The day I put him to sleep, the tumor had ripped open, I should have put him to sleep many day before, yet I loved him too much, and he suffered for it.
So do it as soon as you think itstime and dont wait like I did, It really breaks your heart.