But still no statutory holding period only your intent and how you demonstrate that. 

You can still set credit and income requirements, and as long as you can demonstrate that is your policy then you are not in violation if you deny an applicant with a voucher.  

The true answer is - long enough that you have demonstrated your intent to hold for productive investment use.There's two extremes - If you completed your 1031 and moved in immediately you obviously did not buy it with an intent to hold for investment. 

One final anecdote for this "numbers" section, which is extremely simplified but I think demonstrates my point.

I think millennials are demonstrating the exact same pattern as previous generations.

There are many markets in the country that are growing nearly as far as Orlando and, fortunately for those of us that do some homework, don't get talked about nearly as much (for example, Lexington, KY or Fayetteville, AR or Bloomington, IN).Step two might be to choose a few markets that demonstrate strong growth and browse the available listings on trulia.com.

If someone does a no call/no show, their number/email gets added to a contact named "Blocked Blocked" in my iPhone, which just happens to be blocked....If they demonstrate irresponsible behavior on first impression, why would I ever saddle myself with them as a resident?

There's no statutory holding period but most folks feel a year is usually fine to demonstrate your investment intent.

If you have the evidence to demonstrate the tenants have indeed abandoned the property, do as @Greg H. advises.

I would hate to feel like I am taking advantage of his kindness and would like to demonstrate a proven track record of investment success before accepting an offer like that from him (even though this current offer is admittedly somewhat similar).