At what point do dragons start spawning Skyrim?

Once you kill the first dragon for the main quest, random dragons can start spawning. It’s the second quest in the main questline, so it won’t take you very long at all, and it won’t reveal too much about where the main quest is going.

What makes dragons spawn in Skyrim?

Dragons can attack anywhere outdoors in Skyrim and random encounters are scripted to take place after a certain amount of in-game time has passed without a Dragon appearance. Thus, fast traveling to an outdoor location is a reliable way to trigger a random Dragon encounter.