It depends on how old Snow is supposed to be, too. The character’s age is not the actress’ age, so Regina may have been Regent. A regent rules for a prince or princess who is heir to the throne but under whatever the “age of majority” is in the kingdom. Usually this is 18, but that doesn’t mean it is always 18. So, Regina could’ve started by being Snow’s regent.
Good point she rules until the heir comes of age then she is just the Queen mother.