Alas, “done” isn’t an empirical pronouncement. It’s “best guess” city. Here are five questions to help you decide if big character or plot changes are still needed: If you take the protagonist as he is in the final scene and drop him back into the first scene of the book, will he behave so differently that you wouldn’t even have a story? Did you force your protagonist out of his comfort zone at crucial moments? Has each obstacle pushed the plot and characters forward? Are the consequences of failure dire enough at each stage of the plot? Does each scene in each chapter contribute to its chapter’s overall goal, and does every chapter contribute to the character’s achievement of his story goal?
If you’re confident answering yes to all, you may indeed be at word-tweaking stage and perhaps last draft. Don’t force it. Louis Sachar wrote five drafts of Holes before he sent it to his editor—and that book went on to win the National Book Award and the Newbery Medal.