1. Normally, the shedding skin is wispy and translucent, and is visible in the water much more than out of it. A growth spurt or temps that are too warm, for example, is likely to exacerbate shedding, making it seem more excessive. Is the shedding just from the neck or also from the tail and legs?
2. Time can vary. My RES, when a hatchling, went through a fairly large growth spurt once, and shell a lot, which seemed to be over in a little more than a week. (The only time he's ever done this.) Subsequent periods of shedding have take place more slowly, with just a few wisps of skin hanging at a time.
3. If you notice the seam/lines in between the scutes turning whitish when your turtle is in the water, the shell is growing as well.
4. Just keep the water clean. Normal shedding takes care of itself.

"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-