How many times over?

You're going to have to do a lot of editing.

You're going to have to read your writing over and over again to make sure that it's exactly what you want it to be.

You're going to have to work on it.

How many times you're going to have to edit depends on how you do it.

If you start by polishing before you've made sure your story flows, you're in trouble.

Start with the parts that are going to change the most text. If you have to add a character or a plot element, those are all new words that you're going to have to edit. If you have to change a paragraph to clarify the sequence of events, you're going to have to check it again for grammar.

Start by changing the big things and then you'll be able to focus on the smaller ones. Otherwise, you'll be checking the details over and over again as you change your story.