the time to steep grain is typically not determined by weight. if the volume of grain being steeped is too large to be efficient, break it up into smaller amounts and steep them seperately. for my system 'too much' is about 5lbs or anything that increases the volume in my pot by 1/4.
typically 20-30 minutes is fine for steeping grain as a part of a partial mash brew. much less than that and you won't extract as many sugars from the grain, which is a waste of grain. much longer, and the grains can start to leach tannins and proteins that add to chill haze.
something that can be much more important than the time of steeping is the temperature of the water. I aim for about 150-160 F. the purpose of this is to activate the enzymes that turn starch into sugar so the most sugars can be extracted from the grain.