the GPD rating on the OR/DI
unit is under ideal conditions (temperature and pressure of the supply water). if you have cold water the ro
will run slower. if the water is extremely hot, you can damage or destroy the ro
filters. low line pressure will obviously result in slower production of water. so, the only real way to see how long it takes to fill a certain size container is to actually fill it.
on another note, you should make your new saltwater at least 1 day ahead of time to allow the salt to fully disolve, the temperature to come up to match the tank temp, and the water to "age". simply use a powerhead and a heater in the container that you use to prepare you water.