How long before I see a difference? Day, week, month?

It started working for me within a day. I went from having a constant runny nose and plugged ears to having hardly any runny nose and my ears started to clear, by the next morning after taking my first 5mg dose. However, it also made me really sleepy and my doctor had to cut down the dose to half a pill a day, not a full pill. It still works well, for me, but without all the drowsiness. I can definitely tell when I miss a dose, my nose starts running like crazy again, my ears fill with fluid, etc. I've been told that it depends on the person, so perhaps your does isn't high enough for you? Definitely talk to your doctor if you see no difference.

you said you are undergoing allergy therapy is this using the xyzal or are you doing like I did used something like that and was given allergie testing and injections? hope this helps you please take care

You should see the difference in a day, after taking Xyzal (levocetirizine).

Xyzal (levocetirizine) is an antihistamine that reduces the natural chemical histamine in the body. Histamine can produce symptoms of sneezing, itching, watery eyes, and runny nose. Xyzal works by blocking the actions of histamine.

Xyzal is used to treat symptoms of year-round (perennial) allergies in adults and children who are at least 6 months old. Xyzal is also used to treat symptoms of seasonal allergies in adults and children who are at least 2 years old.

https://www.drugs.com/xyzal.html