First Charge for use to help with the battery meter (to correctly state charge) is recommended by HP to be overnight. They advertise that the battery comes at 60% charge when you buy it.
For current Li-Ion battery technology a good rule of thumb is to get a full charge takes about X 2 the amount of battery life it give you meaning 2 hour battery life, charge for 4 hours. Another thing to keep in mind that at 50% charge time the batter is like 80% charged. The last bit of 20-30% charge capacity takes as long as the as the first 70-80%.
Also about every 20-30 uses or if the battery meter seems whacky/incorrect then recalibrate it with the HP utility or by turning off hibernation/standby at low battery and charging up the battery completely, pullling the AC and running it down completely till the notebook shuts off from a low battery, not the hibernation on low battery condition, then charge all the way back up.
The HP utility will do this all for you seemlessly while you can still use the notebook but you can not turn it off or unplug it until it is done doing its thing.
Other than for the battery meter utility there is no harm in partially charging or partially discharging versus full charge/discharge unlike the older Nickle based technology batteries so just charging for say 2 hours to get an 80% charge for use is fine. You don't have to wait till its 100%.
Now the 2 hour and 4 hour numbers are just ones I pulled out of my a$$e, well actually they are about right (+ or - an hour) for my old n400c, my ZT3000, and my IBM thinkpad at work. Each machine is different but these numbers are probably most common.
Here are some good articles to readLINK 1LINK2LINK3