Way too many variables for any experienced grower to give you a realistic answer,room conditions & plant needs play the largest role in watering.
Instead of trying to figure out an exact amount learn the signs of when to water,since your using soil as the medium you want the soil to dry about 90% between waterings,take your fingers & scarify the top inch or two of the soil,feel the soil for moisture content,then poke your finger down a few inches deeper into the soil,after a few times you'll develop a real feel for when the soil is dry.
After learning how to determine moisture content in the soil then you want to add water generously, until it flows freely from the drain holes.
The reason i allways reccomend the same watering technique to soil growers is this,the water needs of the plants will change on a weelky basis,they will also change drastically when the plants go into heavy bud production,learning to check moisture content is a much better way to be sure you water properly.
This method also has another benifit,every time you scarify the soil to check for moisture you dig up any bugs or larve that are below the surface,once exposed they bake under the HID's & die,this is extremely helpfull when you get attacked by spider mites.