Sex for 12 hours, eh? I hope junior will be a nice little reminder of your wild romp-fest, because unless you'll want to be saddled with another shriekling, you probably won't be romping too much afterwards.
There's a pretty good chance that you would have or will conceive from one instance of unprotected sex, but you say he ejaculated inside you three times? I'd say you have a damn good chance of being pregnant.
I must concur with dalicialynn on this one - if you don't want to be pregnant, then use some kind of protection. Use more than one form. I'm guessing you wanted a baby, or don't mind caring for a screeching, drooling little puke-maggot, so you didn't bother with contraception. Oh well.
I know condoms, birth control, spermicide, etc. Aren't 100% effective in preventing pregnancy, but they still greatly reduce the chances. If you can't get protection, or you're too embarrassed to be seen buying condoms and spermicide, then you seriously should not be having sex.
Get the test done - there's like a little shred of a chance you're not pregnant, but maybe lady luck will be on your side and you'll be baby-free for now.