When you're describing something that takes a certain amount of time, you can describe it in this way. Here are several more examples:

A lot of people can't stand to sit through a three-hour movie.

When you invited me to participate in the conference, I didn't realize that it would be a five-day event.

I signed a two-year contract with AT&T when I got my iPhone, so I can't switch carriers for another 8 months.

Notice that the unit of time (hour, day, year, etc.) is singular. It's "an 8-hour drive", not "an 8-hours drive."