Get some cardboard or other material.

Build box the size of fan.

Cut across one side of the box.

Make a door to go in cut to stop flow of air (until the fan is at full RPM).

Tape a bag to one side of the box.

Tape other side to fan or...

Fill bag will a quantifiable material. Possibly water. Or pellets of some sort.

Either way, time how long it takes for the bag to fill with air from fan.

And viola.