i would guess 2025 as well based on the current update progress. it could happen sooner than that, i don't think its likely it will be later than that. We've only got two biomes left, possibly three if they go a step further with the oceans. I'm guessing they have something planned for after the frost/frozen biome as like a final challenge.

There is NO official timeframe though so take any timeframes you see on this thread for what they are...just guesses by the community based on current information.