Want bigger, stronger forearms? Train them often. Forearm exercises don't cause much, if any, muscle damage, especially for exercises with less eccentric loading like the wrist roller or Thor's hammer (see video). But even wrist flexion and wrist extension will cause little damage.
Furthermore, these exercises have a very small impact on the nervous system because they're very simple and done with light weights. Energy expenditure is low too since it's a short range of motion and smaller muscles are involved.
None of the factors requiring more recovery time are present with forearm training. Unless you go absolutely crazy with the volume there's no reason you can't train forearms every day.
Why Do I Want Big Forearms?
Because big forearms look cool. Hey, they're the only thing that's 100-percent showing in a T-shirt!
Having well-developed forearms will do more for you than simply attracting admiration. Bigger and stronger forearms make it easier to build bigger biceps. Coach Charles Poliquin wrote about this phenomenon 20 years ago. The body hasn't changed since that was written.
By building bigger, stronger forearms you'll be able to handle more weight in both curling and pulling exercises, which will increase the stimulation on the biceps and back muscles.
Bigger forearms and a stronger grip also help the bench press. Look at the top bench pressers in the world; they all have thick forearms. To bench heavy weight you need a strong grip. The harder you can squeeze the bar, the less the wrist will tend to cock and get the bar misaligned. (The bar should be directly above the wrist joint. If the wrist is cocked, the bar moves away from that alignment.)
On top of that, big forearms create a bigger "body" on which to spread the load of the bar. This can decrease stress on the shoulder joint.
One Caveat and a Program
You shouldn't overdevelop one part of the forearms. For example, the wrist flexors tend to be trained a lot more than the wrist extensors (the flexors are involved a lot when curling and pulling) and the forearms supinators are often dominant over the pronators. Just like any other muscle imbalances in the body, this can lead to problems like tendonitis.
So if you want to jack up your forearms and train them every workout (or every day) then work on flexion/extension one workout, do supination/pronation on the next, and do grip work on the third. Then just rotate through that.
Since forearm exercises have a short range of motion, you'll need to do either higher reps or use a slower tempo to create enough fatigue and trigger growth. Sets lasting 30-60 seconds should be your target.
I actually don't count reps when doing forearm work. I put a timer on and keep working until I've achieved failure or close to it in the 30-60 second range. But I'll prescribe a number of reps in a couple of the examples below for the sake of simplicity.
Workout A – Flexion/Extension
|A1||Wrist Roller||3-4||30-60 sec.||1 min.|
|A2||Wrist Curl||3-4||10-12||90 sec.|
Workout B – Supination/Pronation
|A1||Thor's Hammer Pronation (4 second negative)||3-4||10-12||1 min.|
|A2||Thor's Hammer Supination (4 second negative)||3-4||10-12||1 min.|
Workout C – Grip
|A1||Pinch-Grip Deadlift (Hold 30-60 sec.)||3||2 min.|
|A2||Fat-Grip Hold (Hold 30-60 sec.)||3||2 min.|
You can use other exercises if you prefer, but you get the idea.