My belief is that you are "in Business" immediately upon buying your first rental property. You have revenues (rents), expense, Customers (renters), regulations (City and provincial bylaws), taxes and financing (mortgaging).
Each of these items needs to be managed in detail. Choices made, commitments made and contracts adhered to.
That is why we must all treat our real estate investments like stand alone businesses. Each one sustainable by itself, each one you can eventually hire a manager to run the place (property manager) and each one profitable on an annual basis.
Don