Dumb questions are those that aren't asked.
Well it is also a question of which I would say has more than one answer.
Typically I believe it is a calendar year 365/366 from the date of purchase (on your first purchase) subsequent purchases through/via the User Interface should tack on the remaining licence period. If it didn't you could request that it be added via a support ticket.
Those that are on the Automatic Renewal should find it falling in line with their existing license period.
See, clear as mud, so not a dumb question at all. This from an Avast Free user, not having to worry about such decisions.