Pseudoephedrine may be administered as a single dose or a regimen of multiple doses. It has been shown in lab testing that the effects of Pseudoephedrine are both immediate and cumulative in nature and effect. While some patients have reported relief of symptoms in as short as 45 minutes after taking the drug, others have reported that continual administration of the drug has a more positive overall effect. That is to say that taking Pseudoephedrine for more than 1 or 2 days will get more noticeable results.
http://en.wikipedia.org/wiki/Pseudoephedrine
Skip down to the Mechanism of Action section.