If you're using it as a deduction, that means you're deducting interest & taxes on schedule A, & can use the property as much as you want.
If you mean you're claiming expenses against rental income on schedule E, you can use the property for 14 days a year, or for 10% of the number of days it was actually rented at fair market value, whichever is larger. Going over that line means you have to use the vacation/rental home rules, which are a pain in the a*s. You still get to deduct expenses, but you can't claim a net loss on a part personal-use rental.