Here are the actual details from the IRS website on the 15 day rule "Master's Rule". The tricky part is if you go OVER 14 days you must claim ALL income, not just income beyond 14 days, because the dwelling is reclassified.
https://www.irs.gov/publications/p527/ch05.html
Used as a home but rented less than 15 days. If you use a dwelling unit as a home and you rent it less than 15 days during the year, its primary function is not considered to be rental and it should not be reported on Schedule E (Form 1040). You are not required to report the rental income and rental expenses from this activity. The expenses, including qualified mortgage interest, property taxes, and any qualified casualty loss will be reported as normally allowed on Schedule A (Form 1040). See the Instructions for Schedule A (Form 1040) for more information on deducting these expenses.
Used as a home and rented 15 days or more. If you use a dwelling unit as a home and rent it 15 days or more during the year, include all your rental income in your income. Since you used the dwelling unit for personal purposes, you must divide your expenses between the rental use and the personal use as described earlier in this chapter under
Dividing Expenses . The expenses for personal use are not deductible as rental expenses.