Safe Current for Generic 1.8 TFT Backlight

I have some generic 1.8 TFTs as shown in the attached picture. I'm trying to determine what a safe current value is for the backlight, and thus what resistor to use.

Obviously if I power them directly from a 5V external power source they are at their brightest, but I suspect this is bad idea. I'd like to maximise the brightness without damaging the displays.

In the absence of a data sheet (that I can find at least,) I was wondering if anyone had any experience with these displays or similar models and could suggest a reasonable value. 30mA? 40mA? 50mA?

Thanks for any input.