I like the apples to still have a little tooth to them. I test for doneness by spearing a piece of apple with a pairing knife. If it takes a moment to fall off the knife, or I have to nudge it off, it cooks a bit longer. When it holds the end of the knife for half a second before falling off, then it’s done! If you like your apples to be on the softer side, continue cooking until the apple won’t even hold the tip of the knife.