I just bought a 6th generation ipad,how many hours do i need to charge ot before i use it?

You can use it immediately. The newer generation Lithium Polymer Batteries do not have memory, so the old days of having to fully charge/discharge a battery no longer apply. As long as there is charge on your new iPad, use it. Do not however, permit the battery to become fully discharged. This can actually reduce the useful life of the battery.

Charge it as much or as little as you like. You cannot overcharge an iOS device. You can also use it while it is recharging.