I lift heavy and im doing 16/4. How many days can I take off for when my muscles are killing me and I need to have some carbs in order to make it to the gym?
Assuming you mean 16:8?
You can modify your schedule as you wish. If you’re trying to lose weight, having more days with shorter fasts will slow your weight loss. But that’s a trade-off you might be willing to make, to prioritize your lifting. It’s totally up to you.
You might also consider moving your eating window so that you always eat before the gym. For example, if you currently eat between 12-8 PM, and you go to the gym at 8 AM, you could move your eating window to 7 AM - 3 PM.
Or you could modify your schedule to do longer fasts on non-gym days, and eat normally on gym days. Like 5:2 fasting — set your two fast days for non-gym days.
It all depends on what your goals are and what works for your body.