Well, that's a tough question I think. Depends on a person's aptitude I suspect. (Both for the artistic side and the technical if we are talking digitally). How many skills carry over from other areas as well.

For example a draftsman will probably make really awesome town and building maps just because his drafting skills will be handy.

I imagine you mean for someone going from scratch to amazing however. And to that I can offer a couple thoughts that should be taken as worth a grain of salt.

First of all I might just say there are some folks who may never get the knack because they don't have the patience and drive to build up the skills. Possibly they are not great at designing or visualizing, I don't know. It is also art and that tends to be really subjective. What one group of folks thinks is fantastic another will detest. That's just the nature of the beast.

I believe Ascension, who is famous on these boards but sadly absent the last few months, said it took him a couple years before he was satisfied with what he produced. I don't really know what skills he had prior to coming here though, so I'm not sure what his "two years" means.

Speaking for myself, it's taken me a couple years as well to produce what I am doing now. I had artistic abilities but had never really tried to developed them. I had zero experience doing anything artistic on the PC and had never used any kind of digital art package. I learned on GIMP and only about 4 months ago switched over to Photoshop. That is a tool I'm still getting used to. Like you, I still feel like I'm getting better each time. For me, it's a combination of mastering the tools I am working with (oh yeah, I also got a Intous tablet roughly 4 maps ago), learning how to construct a map (Mountains, rivers, climates), and so on. I'm still working on labeling skills. Also, I would say that nothing is old hat to me yet, in other words there is still some portion of experiment in everything I create because much of the time I'm not sure how I'm going to depict everything.

I'm still learning and thank goodness, because if I weren't I'd probably get bored real fast and move onto something else. I think the first map I was pretty happy with was around the 1 year mark. Also, it's worth mentioning that I only get to do this some evenings and when I have free time not taken up with other family responsibilities. If I had more time to spend at this it might have only taken months instead of years.

That's my two cents anyway.