go to the agency and ask.
when i worked for a staffing company, every week i had to fax or drop in my time sheet, signed by the people i worked for. the staffing company paid me once a week - they bill the company that asked for the help.
they'll do a screening sort of interview, and ask what you want to do, etc. they might make you take some tests - typing speed, knowledge of certain applications, etc, depending on what you want to do.
the wait depends on if they have a job to match the time frame and other aspects that you want.