Takes me about, i dunno, 8 hours to do the basic hull detailing - I've started adding noise & grit on what is pretty much a per-pixel basis. Plus i normally do the shading 2-4 times (initalliy, then when the hull lines are drawn, then to accentuate details). and there's also a few hours detailing - especially the machinery details - and also in adding decals. Plus i need to modift each individual decal - triangles, labels, warning strips, etc - by burning, corroding or simply erasing parts of it - before I can finalise the bmp. And then I also need to do shine & especially glow maps, which is often a pain in the arse as I have to select parts of the glow map to create a proper light-=cast effect. Plus I've shifted back to 1024x1024 maps, which makes it a massive job. And my last map I restarted twice because I wasn't happy.
so, overall, i'm guessing it takes between 12-24 hours concentrated effort to draw the maps, depending on size and complexity.
The real difficulty is balanbcing detail, natch - it's dead easy to add too many details and decals, so you end up with something stupid looking. And the problem with HTL ships is that every bit of detail - i.e. little panels or vents on the hull - can look odd in contrast with the extra modelled details.