The number of days you rented the property at full price. If you stayed in the property at all, or rented to someone for less than you could have gotten on the open market, then those days are not included.
I rented my vacation home for 109 days. I stayed in it 15 days. I chose the Tax Court method. Turbo tax wants me to enter 0 under Days of Personal Use which causes my loss to go to zero. Why?