Multiple studies have shown that in 90 minute range, your performance begins to decline without added calories. Obviously this will depend on how well fueled you are before the ride, how hard you ride, how fit you are, and your individual characteristics. I regularly ride 2 hours without eating anything and have gone farther, but that is at a steady pace rather than pushing hard. If I'm going long, I start eating after 40-45 minutes, and keep eating throughout the ride. For most of my rides, I take a break at about 30-40 mile point and drink a Coke, eat some cookies, and chomp some mixed nuts. That's enough to get me home comfortably on a 60-70 mile ride.
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