How much is air mass flow rate of bleed air used for environmental control system in civil aircraft?
EASA and FAA engine type certificates often identify the total bleed mass flow, as a % of the engine's high pressure compressor exit flow. For example, see section 10, "Maximum permissible air bleed extraction" in this for the CFM56-7B, which shows 7 to 13%. If you assume in a civil aircraft that all the bleed air from the engine if for the ECS, you get an answer. (Or maybe you where actually looking for the total bleed mass flow).

If you can get this data from enough references, you could develop a general guideline.