How long should I know a person for before asking them out?

I feel like too much time will end up with us being like brothers, and too little time will end up with the relationship being forced. I get a warm feeling every time my crush(?) calls me, so I may like him, but IDK. He said sure, maybe if we get to know each other more he'd date me, and I agreed. But how long into the relationship before I consider dating him?