I had been offered a break or vacation a few times when my CA19-9 was down to 8. I think that would have been the time to take one, but I was determined to keep up with chemo. After 44 treatments, my CA19-9 was slowly creeping back up when I took the break. I took it because I felt so worn out. I think if I had taken the break sooner it would have been longer. Who knows.
I was going down again but had to miss one chemo due to pneumonia & my numbers went from 29.4 on 10/31 to 93.9 on 12/5 then to 105.2 on 12/19 so no more breaks. I have to admit it was a great 4 months off the chemo: started getting hair, had more energy, could walk better & breathe better!
I do not think breaks are standard. It is up to you & your oncologist.