How long does an allergy attack to pollen normally last?

Days to weeks... If you are continually exposed to pollen, an allergy attach could stretch over days to weeks (in fact, you could be sneezing or wheezing for the entire pollen season).
Hours... The length of an allergy attack from pollen depends on how long the exposure lasts. If someone is exposed to pollen and develops an allergy attack (usually starts with minutes), it will normally resolve within a couple of hours, but could recur with a second wave in the first 24 hours (this is what is called a "late phase" response -- which can last up to 72 hours).