Display Multiplex timing

I am about to start a project which involves multiplexing leds and displays. I want to use a fixed time period, but delay (PWM if you like) the 'on' time of the displays to get an adjustable output. Does anyone have an idea on the timing required to overcome persistance of vision? I figure it's got to be above say 50 or 60Hz tellies use this but the phosphors glow for somtime after further reducing flicker.
Any ideas?

hey i did a project where i was controlling the brightness of 8 LEDs using them as indicators, i had to use a capacitor in order to get a low brightness without the pulsing showing. (that was at 307.54hz approx)

Thanks for that, I think I will get a 556 and mess about abit - thought about programming a pic with a test routine but this way I think will prove the point!