Well, really one could weight train everyday, however, it is usually dependant on a number of factors, such as age, recovery, fatigue, conditioning, etc. The total volume of the work out must be low, generally the intensity is either fairly high (85-90%) or fairly low (30-40%) with rep ranges being 1-5 or 20+ with in those respective brackets. The higher intensity being more for absolute strength gains, while the low bracket being more for prehab/rehab & general conditioning work. Generally the total number of sets per movement shouldn't exeed 2-3, and more than likely the number of movements should be around the same (2-3). Now if you are doing 15-20+ sets per day, I'd say you are or going to be overtraining at a rapid clip and are going to burn out.
Just my long winded way of saying, it depends on alot of stuff!
Hope that helps,
-Andrew