I just got a new HP Pavilion G6-2235us,how do I know when to replace the battery?

Actually every HP laptop is able to download a HP Battery Check(view here), it can provide information about the status of the batteries installed in the computer. It reports on the condition of both the primary and secondary battery, if the computer is equipped with a secondary battery. The HP Battery Check has been proven to be over 99% accurate, that's pretty cool. 

Original Hp G6-2235us 10.8v 5000mAh

To use HP Battery Check, follow the steps below.

• Click Start and select HP Support Assistant.
• On the HP Support Assistant window, select Troubleshoot, and then select Power, Thermal, and Mechanical.
• Click Battery Check to examine your battery and display the results of your battery status check.

HP Battery Check recommends replacing the battery when an internal cell is not charging properly, or when the storage capacity of the battery reaches a weakened state. The performance of all batteries decreases with use and over time. A message indicates where to purchase a replacement battery. HP recommends purchasing a second battery to ensure constant access to battery power.

Source from support.hp.com