That's an interesting question.  I think most Airbnb rentals are less than 7 days, so I think it's obviously "transient."  However, there seems to be an 80% rule, and the question is whether you count the entire property, including the part where the taxpayer has residential personal use.  There is an example in some IRS publication that says for office  in home, it's 39 years, unless your home is in an apartment building that you own and rent out other units for residential use, and then you get to use 27.5 years.  This seems to derive from the last part of Section 168(e):

(2) Residential rental or nonresidential real property.

(A) Residential rental property.

(i) Residential rental property. The term "residential rental property" means any building or structure if 80 percent or more of the gross rental income from such building or structure for the taxable year is rental income from dwelling units.

(ii) Definitions. For purposes of clause (i) --

(I) the term "dwelling unit" means a house or apartment used to provide living accommodations in a building or structure, but does not include a unit in a hotel, motel, or other establishment more than one-half of the units in which are used on a transient basis, and

(II) if any portion of the building or structure is occupied by the taxpayer, the gross rental income from such building or structure shall include the rental value of the portion so occupied.