It depends on a lot of factors. Your body can store about 1500 calories in the muscles and liver with a decent pre workout carb load. I burn about 1300 calories over a 2.5 hr ride so in that case I wouldn't need anything for my normal ride - though If you're lashing away at hills, you'll pour through calories faster and need replacement sooner.
The general rule of thumb i've heard is that for strenuous activity, anything over 2-3 hrs and you start needing supplemental nutrition.