If malignant breast tumor, how fast does it grow?

Depends: Tumors are different for everyone. Some grow fast and some grow slow. Age, genetic factors and ethnicity can contribute. If you are diagnosed with breast cancer, it is important to take time to explore and understand the treatments and options. In general, it can take 2-4 weeks from the time of diagnosis until the first treatment intervention. Be sure you get your questions answered by your dr.

Actually slowly: Doubling time of breast cancers is 100 days, so if you believe cancer starts as one cell ie. Maybe a stem cell transformation and that cell divides to begin cancer growth then::::one cell to 2 cells 2 to 4 4 to 8 8 to 16cells etc it takes 6 years to get to a 1 cm size that is 3/8 of a inch. And that is the threshold of what i can feel mammogram may show it earlier.