I'm in the same boat as szihn - if I'm not modifying anything and just bolting parts together, about 30 minutes for both upper and lower. I might spend another 30 on scope mounting just because I'm picky about getting everything true.
However, I almost never just assemble parts as they are delivered. There are a number of little tweaks I prefer to do when assembling an AR:
- true the upper
- tap for 4-40 screw in rear takedown pin detent hole
- adjust trigger
- make a BAD-lever style bolt release
maybe a couple other things I'm forgetting. It's all easy stuff, but I'll usually spend an afternoon or a couple short evenings getting everything done.
