Never. Writing is a creative process and you need to let your creative juices flow. Don’t worry about the grammar or spelling issues in the beginning for that can be fixed later. If you feel you have a strong message and believe in your characters, then write the best possible story you can write. Don’t rush! Enjoy the moment and get to imagine and visualize what your characters are doing while you bring them alive. After writing your first draft, celebrate and congratulate yourself. Not everybody gets to write and finish a book like you did. Tuck your manuscript away for a month so you can give your mind and eyes a rest. You’ve been working too hard and deserve a break before you dive in and edit.