3 times. If a workout leaves you still very sore in an area rest those muscles some more. Quit often you will find some muscle groups need extra days to recover so you need to be creative. Ideally, you exercise as frequently as you can without getting fatigued from workout to workout. Some people can go back in the gym after one day off, others might need two.
I definitely think it depends on your goal, but I hear a lot of people that do about 3 days a week of weights.
I'm on the Five Factor Plan, so I do weights 5 times a week, but they are very simple and easy with only 10 pound dumbbells. I don't do anything too heavy because I'm not looking to bulk up, as a female. I do cardio 5 days a week and then do about 10 minutes of strength training as well.
I think that one shouldn't weight train more than four times a week. The most important part about weight training is the rest you get afterwards. I know you can get away with rest and working out more than four days by splitting up body parts but I think that's overkill for me. I'm more interested in 3-4 days of total body training, upper and lower body splits or push and pull routines.
Shelton Benjamin -- "Ain't No Stoppin Me Now!!!"