my forearms are naturally big, i do 3 set with biceps every other week, about it...your forearms are used a lot every single day, especially working out in the gym. if i ever decide to compete, that's when i might work them a little harder.
i think it's crazy when there is guys just starting out in the gym, skinny, young, etc..and there in the gym for 45 min working just forearms...i think that is crazy....your forearms get bigger, and strengthens as you go up in weight with other exercises.