Water Heater... how much water need ...?

Few days ago the Gas Company shut OFF the gas line. So we don't have hot water because no gas. They will not turn the gas line for our house until we fix the roof. Roof was damaged due to the tornado. Some body told us we have to drain all water from water heaters (2 Family house), because gas is shut OFF. OK we did this. All water completely was drained out from heaters. Our question is, when we get fixed our roof and Gas Company will turn gas line back ON, how we can know how much water we need put back in the water heaters ? Or we should just open the faucet on the water pipe which is going to the heater and it will be done by its self ? We wouldnt want an overflow of the heater. Please, we need help. Thank's.