My doctor is sending me to a specialist regarding a lump under my armpit that may be skin cancer. how fast can it spread if it is not removed soon?

Tissue Diagnosis: This needs a biopsy to determine if it is even a tumor (benign or malignant). Sometimes other processes can mimick tumors (eg, reactive lymph node, atypical inflammation/infection). A biopsy will tell for certain. Also, in a women with an axillary mass (depending on the clinical appearance) breast imaging and at least a breast exam would be warranted.

Depend on what it is: First thing first. The answer will depend on what the lump really is? See the specialist and have a biopsy done & other work-up then discuss result with your doctor. A lump under the arm pit can be caused by many things- infection, inflammation, cancer both solid and blood calncer etc. So first to see if this is cancer, if it is, then what cancer? Breast vs skin ?Etc, then what stage? Etc. Etc, .

Depends: The rate at which melanoma travels varies. We generally feel that weeks do not matter in waiting, however you should not wait months.