It depends entirely on the person and their spending habits.

Try making a list of what things you will spend money on / buy and then estimate a monthly cost for those items.

Food can be one of the hardest things to quantify, especially if you cook all your own food (a great way to save money). You can try to figure out how much on average you think you will spend on each meal. This is higher for me than other people because I eat a lot of food and a lot of meat. I spend on average $20 per day on food.