If I am discharging a battery at 30% of its energy and then charging it at 0.8C. How long will it take to be completely charged?
1C charge or discharge rate is one battery capacity per hour. As the capacity is given in Ah, Ah/h = A.

If you've discharged 70% of your capacity, and recharge at 0.8 (80%) of your capacity per hour, recharge will take about an hour.