There are no different rules for your first fast. As long as there are stores on your body and Real Hunger doesn't return how long you continue your fast is up to you. If your body runs short of anything it really needs you will know because you will get that return of hunger. Then it is just important to break the fast properly so that the body gets the best quality of food as it returns to digesting what you feed it instead of itself. So you may well have more healing crises but remember how long you fast short of that Real Hunger stage is up to you.