It depends on how many carbs/calories you want to eat. Let's say 2,000 calories and 150g carbs.
1g Fat = 9 calories
1g Protein = 4 calories
1g Carbs = 4 calories
You're eating 100g protein (that's 400 calories).
If you want to eat 150g of carbs, that's another 600 calories.
You're left with 1000 calories for fat.
So, divide that by 9 and it gives you 111g of fat.