How long does a netbook battery need to be charged for?

I have a new asus eee netbook 10 inch screen 1005HA which boasts a battery life of 8.5 hours. The instructions are basic and say to charge the battery but don't say how long. I charged it for 8 hours initially and it lasted only 4 hours. I then charged battery again until it showed a 100% battery full, and I have only had just under 3 hours and was warned that the battery was almost dead. Does anybody have one on these?

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