Check the contest rules.... but I'm pretty sure it's only once.
*edit* an example from the Visit Wolf Creek Sweepstakes....
WGT: The Sweepstakes Period will end at 11:59 pm PT on December 31, 2010. You may play as often as you wish during the Sweepstakes Period, but only one entry per person, per World Golf Tour user account, and per email address will be accepted. Use of multiple email addresses or multiple WGT user accounts to enter is prohibited. Use of a false email address or a false WGT user account, or an incorrect mail address may disqualify an entry. BY ENTERING THIS SWEEPSTAKES, YOU AGREE THAT ALL SUSPECTED FRAUDULENT OR IMPROPER ENTRIES MAY BE DISQUALIFIED BY SPONSOR AT ITS SOLE DISCRETION. Approximate time to play the Game is five (5) minutes.