When does one become a "founder" of a business? For example, would it be when work on the business starts or when the business is incorporated? Such events could be years apart.
"Founder" is a common term used in the startup community.
The SEC refers to founders in its definition of "promoter":
Any person who, acting alone or in conjunction with one or more other persons, directly or indirectly takes initiative in founding and organizing the business or enterprise of an issuer
I think your question is one of semantics. So the answer depends on context.
For example, if you are chatting with your friends, the answer could be when you start working on a new business idea that you one day hope will make money.
On the other hand, if you are talking to VC investors it might be when you file the legal paperwork for your patent(s) or your corporation.
Semantics depend on context.