Approximately how long does it take for a melanoma to go deeper than. 50 mm into the skin. I know there are many variables. I just ask for approximati?

A guess. Melanomas can behave quite differently. Some can stay relatively dormant for years, while others grow like wild fire. I would estimate that a superficial spreading melanoma (the most common type) would reach 0.5 mm in depth in about 6 months to 2 years from its inception.
Widely Variable. Melanoma grows and invades at widely variable rates. It really is not possible to give a useful estimate to your question. As a dermatologist, I have seen melanomas that took 5 yrs to grow that deep and others that seemingly appeared to invade almost immediately. Any suspicious mole should be checked out immediately. Good luck to you.
Can't predict. Different melanomas grow at different speeds. It is not possible to give you a time frame. Some will grow very slowly, while others will grow extremely fast.