Generally speaking, the more often you can target a muscle, the quicker it will improve......So in a perfect world, you would be able to train a muscle group, eat, then train it again and get in multiple sessions per day, the more training/recovery sessions you can get in, the more you grow.....With that said, the limiting factor to this is the body/muscle needs to recover and repair itself before the next time it is targeting, which I have noticed varies widely from person to person. The amount of time you need to recover is dependant on a number of factors, (including age, overall stress level, food intake, hormine levels, and size of the muscle group) and takes more of a feeling out process to determine if you are getting enough rest.....General indicators will be overall soreness...If your muscles/body feels broken down, it is best to take a day off. If you feel like you can workout again, chances are you are ready. Your body has a pretty good policing system in place calledthe Central Governor that actualy limits output when it is overstressed...A good of this is lets say you do legs one day and squat 200lbs for 10 reps, on the next leg workout, you should be able to do at least this. If you can't, its an indication that your central nervous system is still fatigued and is limiting the output of the muscles. Working out in times like this will be detrimental to building muscle (even though its counter-intuitive to what we normally think of toughing it out" or "pushing through it". not the case with central nervous system recovery.
Another factor that determines how musch rest you need is how advanced of a lifter you are and the degree to which your muscled have been strengthened or developed. A relatively novice lifter generally doesn't need as mush time to recover as they are not inflicting a ton of stress on their central nervous system, whereas an advanced lifter bench pressing 500lbs, while he/she can move the weight with the same difficulty as the novice maybe doing 150lbs, the stress inflicted on the body as a whole is much greater, and as such more recvery time is needed.