Appropriate Brightness for LED Displays
It has a lot to do with the environment it will be operating in and the optical filtering (eg. a colored lens) in front that prevents much of the ambient light from splashing around inside the LED display and reducing the contrast. 

As Pedro said, your best bet is probably to test it. 

Above the human eye's flicker fusion frequency (and you'll want to operate well above that or the display will be very annoying) the brightness will be approximately the same for the same average current. So if it's okay at 5mA DC it will be okay with 35mA at 1/7 duty cycle. It's usually not good to go higher than 1/8 for a display if you want maximum brightness or 1/16 if you can tolerate substantially lower performance for saving some parts. 1/7 is a good duty cycle. 

That's a pretty efficient display series as well as a relatively small segment area for each LED so I suspect 5mA average / 40mA peak (1:8) or 35mA peak (1:7) will be more than acceptable for a typical industrial environment given a reasonable colored filter in front of the display.