Cleanroom Classification vs Minimum Sample Duration
Table showing minimum duration (minutes) if you are using 1 CFM (28.3LPM) Particle Counters.
According to ISO 14644-1:2015;
If concentrations are leading to large air sample volumes for classification, a sequential sampling procedure may be applied. You can find it in Annex D.
The volume sampled at each location shall be at least 2 liters, with a minimum sampling time of 1min for each sample at each location. Don't worry about 2 liters since the commercially available minimum flow rate particle counters are 0.1CFM (2.83 Liter per minute; LPM )which will give you more than 2 liters in a minute in any case.