I voted 2 hours as an average but there are a LOT of variables to this question.
Some clients want galls to be simple with maybe one simple graphic and the rest text. Some want multiple designs for a single girl site. Each one featuring custom graphics for each photo shoot. This is a bit more time consuming. Sometimes you also have to factor in the thumb work and linking etc.
There's also something I call designer block which I'm sure every designer has had. If you have it you can't design anything until it passes
Sometimes the ideas just aren't there.