It's a question of how many calories you want to consume over your ride. I work well on about 150-200calories per hour. For me this means about 1/2 bottle of energy drink per hour and packet of Gu every 45minutes for rides over 2hours. This is alternating drinking from the energy drink and a bottle of plain water. Energy drink can get cloying after a while if it's the only thing you're drinking.
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--José
'92 Bianchi Eros
'05 Bianchi Virata
'08 Electra Straight 8 Cruiser