About half of dental assistants have a 35- to 40-hour workweek. Most of the rest work part-time or have variable schedules. Depending on the hours of the dental office where they work, assistants may have to work on Saturdays or evenings. Some dental assistants hold multiple jobs by working at dental offices that are open on different days or scheduling their work at a second office around the hours they work at their primary office.
Source: Bureau of Labor Statistics, U.S. Department of Labor, Occupational Outlook Handbook, 2008-09 Edition