The bulk of their assignments are cosmetic resets that happen twice a year. You can get a lot of hours at that time if you are in an area where there are a lot of stores.
Other than that there is some project work. Dates ranges depend on the client needs. Some have a date range of a week and some are certain days.
Keep in mind that with cosmetic resets they start at 8AM and they end when they are done
Yes there are times that you will have to wait for materials to arrive (pretty common in merchandising) and there have been times that assignment due dates are changed.