You are probably entitled to overtime pay and more breaks than you currently get. You should probably consult with a lawyer about your specifics because it doesn't sound like you are getting everything you are supposed to. For example, you are almost certainly supposed to be getting one additional 10 minute break and you should be getting time and a half after 8 hours of work in a day or 40 in a week. Sending you home early on the last day of the pay period would not be an effective way to avoid an overtime obligation. Sounds like your employer also needs to talk with an attorney. What industry you are in might make a difference but you might want to think about looking into this.
To answer your actual question though, your employer can ask you to stay as long as you are willing to stay. There is no limit to what they can ask IF YOU AGREE AND IF THEY ARE WILLING TO PAY FOR IT. I should also add that certain industries don't allow continuous working for safety reasons - like long haul truckers and rest breaks, that sort of thing.
If your employment relationship is regulated by WA Labor & Industries, you can review the information provided at http://www.lni.wa.gov/WorkplaceRights/Wages/Overtime/WhenApply/default.asp for the overtime information.
Most employment relationships in WA are regulated by L&I.
For information on breaks, see http://www.lni.wa.gov/WorkplaceRights/Wages/HoursBreaks/default.asp .