It'd vary from individual to individual. I prefer a low volume heavy weight approach. Here's my workout from 2 weeks ago (real weights):
Dumbbell Hammer Curls: 15-kilos x 8 reps, 25-kilos x 8 reps, 40-kilos x 8 reps, 60-kilos x 8 reps, 80-kilos (very loose form) x 8 reps (I took a video of this set and added it Facebook)
Tricep Pressdowns on a lat machine: 8 of the stack plates x 8 reps, 11 plates x 8 reps, 17 plates x 8 reps
I measured my right upper arm BEFORE training and flexed it was a little under 19.5-inches.