I haven't found concrete figures, but from what I've read, they do often work more hours during the contract. However, travel nurses in general have far more control over their schedule than regular salaried nurses. A salaried nurse cannot refuse to work, but travel nurses routinely say "No" to contracts if they don't like the pay or the hours.

What happens is they'll accept a few weeks (or 2 months) of long hours, and then take a month off and relax. As you can imagine, if they're getting paid $6000/week, they can easily take a lot of time off and still get paid more annually than their salaried counterparts (while overall working fewer hours per year).