There are still 3 unfinished biomes left at least. IG has given estimates that it should take them about 6 - 9 months per biome. So even with Mistlands slated for 2022, the final release will probably at least still be another year.

But you don't have to start a new world every time. Undiscovered parts of your old maps will receive the new content for their biomes (not a 100% guaranteed, as no entirely new biome has been added yet; but based on previous updates). And you can treat new worlds simply as raiding expeditions; a character's inventory and unlocked recipes travel over to the old world with them. So you can keep all your old bases, and still play the new content.