Just so no one else reads @taxmo 's comment and is confused, there is no "safe harbor" rule that can be relied upon for distinguishing between Schedule C and Schedule E.  Most of the online speculation appears to be based on a 2021 Letter Ruling from IRS which, as always, notes "This advice may not be used or cited as precedent."  

https://www.irs.gov/pub/irs-wd/202151005.pdf

This Letter Ruling presents two fact situations, both based on average stays of 7 days.  Many Airbnb hosts average much shorter stays.  The letter ruling relies on court cases that involved trailer parks.  Consider it for what it's worth, which is not much.  But for now, there's nothing better, other than the always reliable "it depends on the facts and circumstances of each case."