Question: What should you do as soon as you finish your novel?
This is not a trick question.
I know you need to send query letters and do all the administrative stuff of your writing life – this question is not about all of that, it’s about the writing process.
While I have you…it is important to stop working. Recognize that no one ever finishes a novel; they simply stop working on it. It’s OK to say you’re done. Perfection is out of reach. The prose simply, as Dan Barden says, needs to be functioning at a high level.
Here are some wrong answers.
Take a vacation. Go on a reading marathon of all the novels you missed while you were writing yours. Go to grad school. Take up gardening. Take up genealogy. Get a new puppy. Open a bookstore. Build a rocking chair.
Of course you can do any one of these things if you’ve decided to quit writing. That is certainly your call.

Answer: Start the next one.
Here is how the last week of July went for me:
Monday, 25 July – I put the final edits into my first book-length manuscript, backed up the file, and closed my laptop.
Tuesday, 26 July – I sent query letters, which I’d prepared over the months of June and July, to 4 agents whom I’d selected and researched earlier this year.
Wednesday, 27 July – I started writing my next novel.

If you do take time off – stop writing – between novels, and you have not decided to quit writing permanently, be careful. Be very, very careful. The discipline of practicing your craft daily is fragile. Time away is momentum lost and energy evaporated.