It's not specific to Juiced -- it's just the nature of lithium batteries. I've seen different figures quoted by manufacturers, but the general idea is that the batteries last longer if you don't charge them all the way to 100% or let them discharge to below around 20%. Juiced recommends 80% and 20% but I don't think there's anything magical about those numbers. Just try not to charge all the way or discharge completely. If you do charge to 100% on occasion it's not that big a deal.
You'll get an idea how long to charge with a little experience. Generally, if my meter is down to 3 bars I'll charge for about 4 hours. If it's down to 4-5 bars, 3 to 3.5 hours. I don't believe I've gone below 3 bars with a little over 500 miles on my bike.
