Hay fever and extended antihistamine duration

I got hay fever at some point, which now always occurs for 1 or 2 months at this time of the year. I was recommended the antihistamine cetirizine 2 years ago, and I still have the same small package. Because I didn't need many pills yet. What's strange is that I seem to only need to take 1 pill and then I have no symptoms for many days, like 1 or two weeks or even longer. The Internet says a pill is expected to work for 24 hours. I wonder if the unexpected long duration in any way has to do with being a POIS sufferer, as some people have found relief with antihistamines. Maybe it's not related, but IMO worth mentioning. What do you think, and did you notice the same?