How long does it take for pet hair to go away?

I'm mildly allergic to pets (likely their hair) and get cold-like symptoms. Usually it isn't bad, since I take an allergy pill when I visit somebody who has pets and I'm just fine, but the apartment my family moved into (in late March) had a lot of pet hair in it. We vacuum and leave the windows open regularly, but I'm still fairly allergic. How long does it usually take for pet hair to leave? Are there any tips to help with the symptoms, or to get rid of the allergen faster?