The established flow rate for compliance testing in the HSE’s Analysts’ guide HSG 248, 2005 is currently 1 litre per minute.
The new version of HSG 248 will contain guidance on a range of flow rates for personal monitoring which aligns with the WHO The Determination of Airborne Fibre Concentrations: A recommended method, by phase-contrast optical microscopy (membrane filter method) published in 1997, states in section 3.1.6 Flow Rate:
The flow rate used in sampling should be in the range 0.5-2.0 litres. min-' when comparisons are to be made with fibre-concentration limit values defined for 4-hour or 8-hour reference periods. Flow rate should be adjusted, wherever possible, to give fibre densities in the optimal range for accuracy and precision.
WHO | Determination of airborne fibre number concentrations
You can read more at https://www.who.int/occupational_health/publications/airfibre/en/.