If you can train a muscle group hard, with max intensity on most weeks, you'll need a week to recover, or at the least, several days. Everyone is different and recovery will vary from person to person, but I'd suggest that you try to work hard enough to require 6-7 days off that bodypart. Smaller body parts tend to recover quicker, but they are also used more frequently. Biceps and Triceps are used on arm day, as well as in back, chest and shoulder training. It can take time, years of training, to learn how to bring max intensity and even then, you can always learn more. So look at your workout. Be honent with yourself. Ask yourself, Are you really working hard enough? Are you really giving your best effort? Then allow the muscle time to recover and be strong, so that you can bring your max effort again next wk. 

I tend to work with more volume. I lift heavy, but I do a lot of sets. An average quad day takes me 3 hours. An average back day takes me 2.5-3 hours. Sometimes, I will back by intensity down and allow myself some rest, but on most workouts, I give my all, on or off gear