If you use your vacation property as a home and rent it for fewer than 15 days a year, you do not have to report any rental income on your tax return, nor can you deduct any expenses as rental expenses.

If you use your vacation home for personal purposes and rent it for more than 14 days per year, you generally must divide your total expenses between the rental use and the personal use based on the number of days in the year it is used for each purpose. You can deduct certain rental expenses against your rental income, but you can not deduct rental expenses in excess of your gross rental income. However, if you itemize your deductions, you might be able to deduct mortgage interest, property taxes and casualty losses on your vacation home.

For more information, see http://www.irs.gov/taxtopics/tc415.html.