You are talking about the PWM frequency. If you are having issues with that make sure to look for a manufacturer that has theirs at 30 Hz or higher, when I worked with LED's we tested that 50 Hz was enough to make sure that humans were unable to see that the light was not constant on.
I've built LED displays and the lowest frequency at which I wouldn't notice flickering was 500 Hz. This was at 1/8 duty cycle though, so you might get away with a lower frequency at a higher duty cycle.
I find your claim that 50Hz was enough rather strange. Even incandescent light bulbs have noticeable flicker at below 50 Hz and those have considerable warm up / cool down times, as opposed to the LEDs.
