How long does it take to know that a pet allergy is not going to get better?

Likely Not Soon: If you are truly allergic to animal dander like cat and dog it is unlikely to get better. Allergic sensitization can change with time, but don't expect abrupt resolution overnight. See an Allergist to confirm sensitivity and treat with nasal sprays and antihistamines. Avoidance also key. If you have pets limit access to bedroom, HEPA filters, frequent washing. Allergy shots can also be helpful.