I Have A Balance On Battlenet Account Was Wondering How To Use It To Buy A Month

After doing some research it does not seem that at this time you cold buy game time with your battlenet balance.

"Battle.net Balance is a new feature that allows players to add funds to their Battle.net account. The balance can be used to buy Blizzard Entertainment products and services directly through Battle.net such as World of Warcraft character name changes, realm transfers services, and other paid services; as well as digital versions of select Blizzard Entertainment games including Diablo II, StarCraft II, and World of Warcraft digital upgrades.

Note: At this time, Battle.net Balance may not be used for Blizzard Store purchases such as digital pets, mounts, or apparel and collectibles.

In addition, with the upcoming launch of Diablo III, players in certain regions will also be able to use their Battle.net Balance to acquire items from the game's currency-based auction house. Diablo III players will also have the option to increase their Battle.net Balance by selling items in the auction house, and using that balance on future auction house purchases or to buy Blizzard products. For more information on the Diablo III auction house, read the Diablo III Auction House FAQ."