This information is sourced directly from the IRS Website: Here
Rental Property / Personal Use
If you rent a dwelling unit to others that you also use as a residence, limitations may apply to the rental expenses you can deduct. You're considered to use a dwelling unit as a residence if you use it for personal purposes during the tax year for more than the greater of:
14 days, or
10% of the total days you rent it to others at a fair rental price.
It's possible that you'll use more than one dwelling unit as a residence during the year. For example, if you live in your main home for 11 months, your home is a dwelling unit used as a residence. If you live in your vacation home for the other 30 days of the year, your vacation home is also a dwelling unit used as a residence unless you rent your vacation home to others at a fair rental value for 300 or more days during the year in this example.
A day of personal use of a dwelling unit is any day that it's used by:
You or any other person who has an interest in it, unless you rent your interest to another owner as his or her main home and the other owner pays a fair rental price under a shared equity financing agreement
A member of your family or of a family of any other person who has an interest in it, unless the family member uses it as his or her main home and pays a fair rental price
Anyone under an agreement that lets you use some other dwelling unit
Anyone at less than fair rental price
Minimal Rental Use
There's a special rule if you use a dwelling unit as a residence and rent it for fewer than 15 days. In this case, don't report any of the rental income and don't deduct any expenses as rental expenses.