So, question for people with past experiences with this jam! What would you say it's an appropriate game length? The fact that the point of metroidvanias is to reward exploration and backtracking asks for a bit of a long game, but I'm also aware that it's very very unusual for jam games to be played for longer than 10-15 minutes, so...

A preferred length is one where I have fun for as long as I play. Whether that’s 5 minutes or 30 minutes depends on the design and feel etc.

Our stretch goal will be about an hour of gameplay that the player would actually want to play, but that is a pretty big scope even for a team of our size (6 contributors) for the length of this jam. I'd say just try to make an engaging experience and expand it as much as it feels right to do so. If your game is intriguing enough, people will probably want to play a bit more of it!

This pretty good article covers this https://itch.io/blog/490276/so-you-want-to-make-a-metroidvania-month-entry

(in terms of biomes, upgrades, bosses, enemies...)