Really depends on the health (& body weight) of each individual snake. One thing to keep in mind: you didn't mention how often you're trying to get him to eat a rat, but offering more than once every 5-7 days
tends to work against you, as it stresses the snake. Personally, I'd get some "used" mouse shavings from a local pet store & use it to scent the rats so they smell more like mice.