The dosage is 24hr so it should build up and cover the cat the entire time, same as a glucosamine supplement would, they say to start the zylkene before expected stress so i wouldnt expect an instant change (from what i read in my box anyway!).
I saw no change at all when i gave zylkene and i gave it a fair trial of 3-4 weeks! I think alot of it can be the effect giving it to the cat does to the owner, a calmer owner = a calmer cat.