Make notes right away.
When you’ve just wrapped a project, don’t wait to recover before making notes on what you could have done better, what you’ll change for next time, what you thought went well. Make them now, while you’re still exhausted.
While you’re still in it.
Wait a couple of days (that often turn into a couple of weeks), and suddenly the details aren’t as fresh. What plans worked, and what approaches needed the slightest tweaks, are no longer fresh in your mind.
Wait and risk not capitalizing on the knowledge you just gained.
Make those notes now, and be rewarded later when your next project benefits from all the perspective you have right now.