When wholesaling vacant residential land (city lots or larger residential parcel in rural areas) how long is the due diligence period?

To me the biggest issue with out of state investing and long term rentals is that you have to deal with property damages and/or evictions. 

It is a 3 unit airbnb portfolio with each unit in a different city.

The return on equity is probably very low considering they are paid off so maybe explore refinancing or selling the lowest performing one(s) or your biggest headaches to reinvest in more lucrative deals.I noticed you also used a metric of number of rentals to attain your goal rather than $$$/month.

I'm based on the west side of the Twin Cities and am investing on MN's North Shore (Duluth, Two Harbors, Grand Marais) The house were under contract for is a long-term duplex. 

Now that people are starting to socialize again and this city is opening back up soon, I would imagine that many renters and visitors will be coming back to SF in the next year or two.

Conversely, the number of small-multi family properties being built is a tiny tiny fraction of the number built from the 1950s-1980s in cities like Phoenix and Mesa.

Remind them of quiet hours, city ordinance, right to quiet enjoyment, etc.

That was my biggest mistake.

That is, buy one home in city X (where I don't live), live in it a couple of months let it be an STR the rest of the time and then the next year (or in two years) repeat again in the same city.