Last MVM, I came in halfway and still had time to finish the game, so its definitely possible to start from scratch now and finish in time.
As for game length, there's no real requirement. Last month there were some short entries that were really cool and polished, and there were some entries that were longer. As long as it fits the metroidvania criteria, you'll be fine.
Personally, this year I'm focused on implementing better player controls and abilities using FSMs as well as learning tools from my engine that I haven't learned yet. My game will end up being shorter than last time, but that's just a personal decision based on what I wanted to gain from this jam.