It depends on a lot of things. If you've never done a skin for a plane before, it can take a while to figure out the boundaries inside the skin, how the panel lines match up, and all that. My first 205 took several months of on and off work. However the second one (once I had all the hard work out of the way) only took about a week. The third even less, but that's because it used a lot of the work from the second one.
On and off, I think a lot of skinners her put many months into their skins. They want it to look good. That takes time. Consider, however, that we're not working all day on them. If we did they'd get done a lot faster.