Allergy symptoms typically last the entire season that your allergies are seasonal too.
If the pollen or other things that trigger the seasonal allergies are still in the air and you're still exposed to those particles or other allergy triggers then the allergies will last until the allergy triggers are gone.
You can take some allergy medicine and antihistamines to help with the allergy symptoms and you should begin feeling better within a week or 4 months at most.
But as long as the allergy triggers are still around the allergies won't fully go away until the allergy triggers are gone.
You can also use an air purifier in your home to help clean the air of the allergy triggers and keep the air clean for you to breathe.
If the allergies continue to last longer than the seasonal allergies are around then seeing the doctor about it would be a good idea.