How much should I eat when cycling?

When cycling, well-trained athletes should eat approximately 30 g of carbohydrate/hour for 1-2 hour rides, 60 g/hour for 2-3 hour rides, and up to 90 g/hour for rides lasting longer than 2 ½ hours [1]. Beginner or aspiring cyclists typically need less, but can benefit from eating 30-60 grams of carbohydrate/hour for rides lasting longer than 90 minutes. 

You should begin refueling around 45-60 minutes into a long ride. Many cyclists prefer to split their intake into 20-minute increments to minimize GI discomfort.