Once I start lifting again (I hate you, discs) I'll increase the weight as often as I can. I don't see any reason to wait around to up the weight, so long as you safely completed your reps the previous time, while being humble (i.e., no cheating or forced reps, proper form, etc.). I'd only make an exception if I was juicing (which I will never do) simply because the gains would come so quickly, I'd be afraid of going too fast and leaving my tendons and connective tissues in the dust.
Beginners should also be wary of jumping too quickly in order to avoid this. Then again, Rippetoe has people adding 100 lbs. to their squat in a relatively short time, and they seem fine.