There is no cookie cutter answer to that. It varies. I had mine for a good 2 years before I ever even went to get it checked. It turned out to be pretty deep and a level V - but I had no lymph node involvement or mets as far as we know. The best thing is to get anything that changes checked asap. And that doesn't mean just moles. My melanoma did not look anything at all like a mole! So just be aware of your skin, note changes and get them checked.
I wish you the best. Don't panic - try not to even think about it until you get the results and see what you are looking at.