For what length of time will antihistamines remain active in the body?

Depends . It depends on the antihistamine. There are fast acting antihistamines such as Benadryl (diphenhydramine) and Atarax also known as Diphenhydramine and Hydroxyzine that generally last approximately 6 hours. Longer acting antihistamines such as Allegra and zyrtec also known as fexofenadine and Cetirizine generally last approximately 24 hours.
Depends. It depends on a few factors including the rate at which the antihistamine is metabolized by a particular person, other concommitant medications which can affect the rate at which it is metabolized, age of the person, and whether an antihistamine is long acting( like zyrtec) or short acting(like benadryl).