Just go as long as you can. Don’t eat anything sweet during your eating windows. Eat low carb fruit like blueberries, strawberries, avocados if you have intense sugar cravings. Read the about page of this subreddit.
Don’t beat yourself up if you can’t complete your goal. It’s ok to listen to your body and work your way up to a longer fast. I worked my way up to 48 hours. Are you doing a water only fast or will you be drinking plain tea and black coffee as well? Make sure to include electrolytes because you will get super tired.