If someone is leasing their second home to tenants, how much leasing needs to occur before it is considered an investment property?
100% of second/vacation homes are investment properties.
If you are referring to what makes it a non-owner occupied or owner occupied rental property then if the owner stays in the home 14 days out of the year then it is owner occupied rental property and you have to refer to 1026.3 and its Commentary for the requirements under Reg Z. However, credit extended to a non-natural person is exempt from Reg. Z.
If the credit is extended to a natural person then you have to determine if the loan proceeds are for a business or consumer purpose. If for a consumer purpose the loan is subject to Reg. Z regardless of the collateral.
The opinions expressed are mine and they are not to be taken as legal advice.