There's no set amount. The size is set based on the style of flying you want to do with the model. But a couple of rules of thumb for basic styles are about 10 to 12% chord for full span strip ailerons when used on trainer and sport models. For regular ailerons about 15 to 20% of the chord for the outer 1/3 to 1/2 of the span will work fine.

Just don't look at the hardcore 3D and similar models as a guide. Those designs are intended to try to turn themselves inside out at low flying speeds so you often see full span ailerons of 25 to 40% of the chord.