Re: melanoma how long has it been there
Hello and welcome!
First and foremost, thank the powers that be this cancer has not spread to your lymph nodes! While melanoma is no fun whatsoever, having it spread to the lymph nodes - or beyond - truly complicates things.
As to how long this has been on your arm? That's really not all that easy to say. Melanoma is not the most common of skin cancers (squamous and basal cell are), however, it is far more aggressive and can grow fairly quickly. Melanoma can develop as a new "mole" or it can "take over" an existing mole. It's almost like a creature all its own - it's why a lot of melanoma patients call it "the Beast."
In my particular situation, I noticed a faint freckle on my left knee that hadn't been there before in late October/early November 2006. Why I noticed it on a body filled with moles/freckles I've no idea. A couple of days later, when drying off from the shower, it appeared a little darker than it had before, so I decided to watch it. (I am certainly in the risk category for melanoma, so I am cautious anyway) Over the next month, until late November 2006, this freckle darkened and grew until it became the size of a typical mole - which is what I thought it was. In early December 2006, it sort of "popped up," which caused a bit of concern. It also looked "weird:" perfectly round, crisp border (as if some one had drawn it on my body), very dark, and was about 1/8" in diameter. I was scheduled for my annual physical in January 2007, and my doctor recommended we remove it. I had it removed on February 21, 2007, and was informed it was melanoma on February 23, 2007. It really hadn't changed since December 2006. The final diagnosis was .7mm in thickness - so that was since early November 2006, a total almost 4 months. My melanoma was nodular, which meant that after appearing on the surface it then began to grow down into the skin, as opposed to along the surface.
I think it was easier for me, because my melanoma didn't develop from an already established mole that started to change. So, I had more of a history of how it started and how it developed, and the amount of time that elapsed. And it was highly visible for me. If this had been on my back on my scalp, I doubt I would have picked it up as quickly.
So, the answer to your question? It's really difficult to say. Melanoma is too aggressive to live with for a long period of time. But I don't think one can say that within 6 months, it's stage 1, within 1 year, it's stage 2, and so on. There has been some research that the body's own immune system attempts to fight off melanoma (in fact, there are clinical trials that address this in a more sophisticated version), so it could be that, depending on one's health, if one has a melanoma, the body may attempt, sucessfully, to fight it off for a certain period of time. This is purely speculative, however.
If your question is stemming from some idea that perhaps you should have known earlier, then don't beat yourself up. Some people who have late stage melanoma (stage 3 or 4), have never found a primary (the initial "mole") and it was a very rude awakening to discover they had melanoma - in some cases a not so good prognosis. You have caught this at a good stage, no lymph node involvement, even though you will need ongoing care. You are lucky, and take future precautions from this time going forward!