It depends.
I go to a school with uniforms, so dressing for school doesn't take me that long since I don't have to pick anything out. However, I'm also required to wear tights under my school kilt, and if you've ever worn them, you know putting them on can be a pain in the behind. Regardless, dressing for school takes me at the most about two minutes.
It's the same if I'm just sitting around at home. I'll just pick whatever's on the top of my drawer and throw it on. That only takes me a minute or so.
If I'm going out in public, it'll take me a good ten minutes to get dressed--not including hair and makeup (Though frankly, I never really do anything with my hair besides brush it).
If I'm going somewhere formal, it'll take me at least fifteen minutes--again, not including hair and makeup.