Any food is going to take at least 15-30 minutes to break down and get into the bloodstream and how long it last depends on too many unknowns to give a good answer, but if you intervene with HC food due to your cat dropping too low, you want to continue testing for 2 hours from the time you had to intervene to make sure the BG stays up.
Simple sugars like Karo/honey/syrup get into the bloodstream faster, but they also wear off faster. You should see the BG rise within 15 -20 minutes but you should follow up with something else that will last longer.