Mine takes exactly 3 and a half minutes, so I guess it's crap. Well, I knew that anyway.
I wondered if it was possible to work out the actual free air displacement based on that timing. 50 litres at 8 bar is 400 litres, 400/3.5 is 114 litres per minute, so almost exactly 4 cfm. That seems about right, given that the manufacturer says it's 7.34 cfm. Does any of that make sense?
172 ltrs per minute. filling a 50 ltr tank to 1 bar pressure takes around takes around 17 seconds, to fill to 8 bar takes 8 times as long. 8 x 17 = 136 seconds or 2 mins 16 seconds.
one thing you have to bearing mind is manufacturers state different cfm measurements, one is displacement the other is free delivered air, on a 3hp compressor displacement is about 14.5 cfm and free delivered air is just over 10 cfm. FAD or free air delivery is the one you want to work out the fill time of the tank