How long does it take before I feel a lump?

Here's some of the questions that might come up in the clinic relating to breast cancer detection.

Why do I have to get so many uncomfortable mammograms?

How long does it take for breast cancer to develop before I feel a lump?

Robbins Pathologic Basis of Disease elegantly answers:

"One can begin the consideration of tumor cell kinetics by asking the question: How long does it take to produce a clinically overt tumor mass? It can be readily calculated that the original transformed [cancer] cell (~10micrometers in diameter) must undergo 30 population doublings to produce 10^9 [one billion] cells (weighing ~1gm) which is the smallest clinically detectable mass.

In contrast, only 10 further doubling cycles are required to produce a tumor containing 10^12 [one trillion] cells (weighing ~1kg), which is usually the maximal size compatible with life."

Sadly, the corrollary to this is that by the time a solid lump is felt in your breast, the cancer has already been doubling for 30 generations BEFORE IT EVEN BECOMES A CENTIMETER IN SIZE, giving it plenty of time to evolve plenty of ways to evade your immune system, develop resistances to drug treatments, etc.  That's why surgery, radiation and chemotherapy are often done in tandem to reduce the risk of spreading.

The good news is that mammography can spot smaller lumps, before they can be felt by physical exam.  The bad news is that mammography is an imprecise image of the processes going on in the breast which leads to the occasional false-positive (getting an unnecessary biopsy) or false-negative (lump is not very suspicious and the diagnosis of breast cancer is missed.)