The dragons never stop spawning. You can easily test this by just travelling to various dragon nests, you can kill dragons at those spots over and over again.
Why won't dragons spawn Skyrim?
You need to finish the 2nd battle in the main questline 'Dragons Rising' you need to do this to get your shouts this is the ONLY way to get dragons to start to spawn.
Do dragons still attack after killing alduin?
Based on experience1, killing Alduin doesn't kill all of the dragons, so you will still find dragons roaming the province of Skyrim on your next adventure. The goal of the game wasn't to just get rid of ALL2 of the dragons.
