How quickly can a tumour show up between scans?

Paying for scans is pretty standard, I live in Toowoomba QLD and there are 2 providers here. One is slightly less expensive than the other but $1200 is the quote for mammogram, US and biopsy and is around about what I paid when I was first diagnosed 2 1/2 years ago. Without the biopsy its about $400 a side.
Obviously there is medicare rebate, so we end up $530 out of pocket but still need to pay up front and then claim the refund which is sh!t.
@arpie FA is fibroadenoma - benign tumour. I am 44