when my f veiled was young she would shed every 3 to 4 weeks. After about 6 months (she is now a year) till now she sheds maybe once every few months and only in small sections.
It depends, older chameleons usually don't shed big peices, but younger usually do. Small spots of shedding could be bad depending on how small they are and how long they have been that small. The smaller and longer a shed takes, the more likely your cham has not gotten enough moisture for proper shedding.
Timewise, I think its about once to every other month for younger chams and then slowly longer and longer for older. I figure it also depends on the breed.
My younger ones shed every 3wks or so and my older guys not as often. When they all shed they shed in big sections and usually by the end of the day or overnight are done with the shed.