How much will definitely depend on lifestyle. My son in phd program - doesn’t drink. doesn’t go clubbing - so spends very little on social entertainment.
He doesn’t have a car and we pay for phone and Netflix. He’s still covered on our insurance. He doesn’t cook much but still doesn’t spend too much on groceries - lots of peanut butter and cereal. He goes out for lunch with other students. Never buys clothes or items for his apartment. And assures us he never needs anything. Nalurally frugal.
He lives well under his phd stipend and banks a lot of his money.