How to find sampling time for operation of a real time system?

I have a LEGO Mindstorms EV3, I have designed a algorithm to control the lego robot in upright position, basically i have made a two wheel inverted pendulum, so while running the robot through "Run on Target Hardware>>External Mode" i have taken a sampling time of 0.01 sec, well i want to know what is the basis for taking the sampling time or sampling rate for such real time system? If we do trial and error than i get to know that 50 samples i.e 0.02 sec is also sufficient, below it doesnt seem to work properly... so can you please suggest, some guidelines for selecting sampling time...