critical hits only occur when you have a weapon equipped that's capable of critical hits.
if you go to the blacksmith and view the weapon attributes, the weapons capable of critical hits have a little cracked stone icon at the bottom of the listed attributes. other weapons have special attributes as well. the candle has chance of fire (hence the fire symbol on the attributes) and the lightsaber has chance of electricity.
not sure what the % of chance for crit damage is, but if you have the crit damage weapon equipped, ive found it to hit a couple of times every hundred hits or so.
its never a fixed number or completely random. its all the programming and math of the proc.
say a weapon has 30% chance to do critical damage. every time u hit an enemy with that weapon a program generates a number 1-100. lets say that if the number lands 1-30 it gives a critical hit. if it lands on 31-100, it does not. thus giving you a 30% chance to hit with critical damage.