How close your light can be depends what kind of light setup you have and if its air cooled or not. There is no real "average" of how tall a plant should be, because it will vary with how long people choose to veg as well as the genetics of the strain. My rule is that I try not to veg past 1/3 of the availible space between the plant top and the ceiling. I use a lot of LST to overcome my relatively short height of my grow room. Basically, unless its an Autoflower, you grow it however tall you want and then switch the to flower. The basic idea of course is that more veg = bigger plant = more buds. The downside is that the longer you veg the more increased chance that something could go wrong. YMMV.