The snake will start to look dull, faded out. The eyes will start to turn cloudy and eventually have a milky white look to them. As the shed process progresses the eye will turn normal looking.
At this point your snake could shed any day. I've had them take as long as a week after the eyes cleared up.
From beginning to end the process can take 1 to 2 weeks.
They shed as they grow. During growth spurts I've had snakes shed every 3 weeks. This lasted 6 months.
I have adults that shed 2-3 times a year.
Hope this helps.