Of course you need some sort of documentation in case of an audit. That is true with EVERYTHING on the tax return; you need documentation in case the IRS questions it. If the OP was seeking tenants, there should be some sort of documentation of that (listings on Craigslist or whatever other means they were seeking tenants).
But the fact is it WAS a rental as long it was available for rent and seeking new tenants. So whenever that ended, that is the date it was converted to personal use and that date is what is used to determine rental expenses and personal expenses.
