How long do I need to treat conjunctivitis for?

Depends. If viral, it will usually last 2wks and will resolve. Bacterial conjunctivitis needs to be treated with antibiotics for 7-10 days depending on bug, sometimes longer. Allergic conjunctivitis can be seasonal and needs to be treat longer; same for vernal conjunctivitis.
Depends. Depends on the organism causing the conjunctivitis. Viral, bacterial, fungal, etc. Usually 7-14 days should be adequate.