Like sunsanvil mentioned, it varies greatly.

I found that when I started off I only focused on actually constructing the model and didn't think to paint them. I ended up finishing models pretty quickly - maybe 5-10 hours max.

As I progressed and my skill level improved, I started incorporating more techniques (painting, applying decals, weathering) and being more pedantic about the final finish. Accordingly, I now spend more time per model - definitely 30+ hours per build.

Depending on how much time you are willing to invest in the hobby will determine the build time. I would recommend spending your first few builds focusing on the construction of the model and then move on from there.

Have fun with it - it is great fun once you get into it.