[ThrowawayR2]

That depends on what you think the title of software engineer means. If you think of it as meaningless, then there are no requirements. If you think that the title of software engineer should be like the professionally licensed engineering fields, then look at what the requirements for professional recognition or licensing in those fields are. Usually its agreeing to abide by a code of ethics, proving that you possess understanding of the fundamentals of your field (either through education or learned on your own), demonstrating your competence though a number of years of working experience, and demonstrating that you're keeping up to date with changes in your profession (the continuing education requirement).