1. You don't ask the temp agency anything about giving notice until you have a firm offer, start date, rate of pay, etc. If they get even a hint that you might be looking, they may try to make it easy on their client by replacing you with the first available person.
2. Once you have that job offer, then you will need to think about:
-- how soon does your new employer need you? If they insist they need you right away, you go. If they can wait, gauge how comfortable they are with waiting the two weeks that will keep you in good graces with the temp agency.
-- did you sign any paperwork with the temp agency committing to giving a certain amount of notice?
-- does the temp agency lag your pay? Is there a possibility they'd withhold a check for you leaving them in the lurch?
-- how much do you care about keeping good relations with the agency, whether for a reference or for possible future work?
Since the job duration is longish, the usual two weeks notice is expected. It may not be required, since temp agencies do know that people use temp jobs as filler until they find permanent work.
When you do approach them, if you want to leave sooner than 2 weeks, offer as much notice time as you're willing, then politely ask if you can leave in a few days (or immediately) instead. If possible, offer to go in and wrap things up or train a replacement.
Good luck with getting one of the permanent jobs!