Some of the major factors in how much recovery time that will be needed is the amount of volume (for that body part) in the workout and the intensity at which you are lifting. To simplify things, we will look only at volume for now to see how the "48 hour rule" is really just a minimum rule of thumb and not gospel
Your question pertains to an arm workout. If you are doing a body part split and have an "arm day", then you are probably performing too much volume to recover in 48 hours. Also, if you are doing a body part split and have an arm workout and are looking to do arms two days later, that means that you must be neglecting other parts of the body if you are doing arms again so soon.
The 48 hours usually is sufficient for total body workouts. In those, the arm part of the workout is small and often no arm isolation at all. In upper lower, you may do a few more sets for the arms, but your volume is still not too great, so doing these twice each for upper and lower is usually not too much of a stress. In body part splits, you are often doing 3 to 4 exercises per workout for that body part. You need more time to recover because you are doing more volume. These should be done once or at most twice per week.
For example (again, to keep things simple we'll assume same weight and reps used across all sets), in a fullbody workout, you may do 1 exercise for biceps for 4 sets 3 times per week for 12 total sets. In an upper/lower, you may do 2 exercises for 3 sets, twice per week (12 total sets). In a body part split, you may do 4 exercises of 3 sets each for biceps (12 total sets). Your weekly volume is the same but the time needed to recover from that volume depends upon how condensed that volume was.
On the flip side, if you like to do one set of pushups daily, you probably don't even need to worry about days off. The volume and intensity is probably light enough to consider these as nothing
more than active recovery.
Recovery is also an individual thing, so even if you follow the volumes listed for the various splits above, there is no guarantee that you will recover in that time or you may recover sooner. Experience is the great teacher there. As far as soreness, it is not always a perfect indicator. I've known some people who are perpetually sore and I've known people who almost never get DOMS. For them, it is not a good indicator of recovery. If you are just starting out, until you get some experience and learn how your body reacts to exercise, take the cautious route. If you are sore, do a light recovery type workout instead. Do some mobility drills and callisthenics. Do another day of cardio (or extend the time if it was already part of the plan). Postpone your workout for a day. It won't hurt you. At worst, it puts you back a day on your schedule whereas pushing yourself beyond your limits can cause an injury that will put you of the gym for many days and in some cases causes people not to go back at all.
Last edited by Depalma; 04-03-2008 at 10:55 AM.