How long should I let a first draft rest before I start editing?

Long enough that you read it as a stranger would, and for most people that's about 3–4 weeks — enough that you've stopped hearing the sentences in your head and can catch what's actually on the page versus what you meant to write. Two things make the wait productive rather than just idle: don't touch the manuscript at all during it (no "quick fixes"), and use the time to start something else so your brain isn't circling the same book. When you come back, read it once straight through without editing, just taking notes — that pass shows you the structural problems, which are the ones worth fixing before you polish any prose. If you edit line-by-line first you'll lovingly perfect scenes you later cut.