Yes - you probably have reduced your supply, but that is the goal of block feeding :) and it doesn't mean that you have reduced it to a level below your baby's need. It sounds like now you are a perfect match between supply and demand. If baby is not maintaining a growth curve, or if he seems hungry I'd offer both sides. Otherwise it sounds like you're doing great!
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I also block fed (10 hours on one side!) until my baby was about 7 months. He still only takes 1 breast a feed usually - and tells me if he wants the other side. And last week I realized that if he goes 5 hours without nursing (he got busy and I forgot to remind him) I still leak! Oops! 
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