Flow is the quantity of water passing through a hose in a given unit of time; it is measured in litres per minute (l/m) or per hour (l/h).
To measure the flow of your system, all you need is a chronometer and a recipient with a known capacity, for example a 5-litre jerry can.
Open the tap as much as possible, then measure the time required to fill the can, then make a simple calculation. If the 5-litre can fills in 10 seconds, water flow is one litre every two seconds, i.e. 30 litres per minute.