When Zoey was ill, the vet said that within 12 hours, levels are often high enough to see some improvement. That was twice a day medication, so I imagine it would be 24 hours with once a day.
However, in reality, I think it took 3-4 days for them to kick in at all for Zoey the first time. After being taken off and put back on a different antibiotic a few days later, she hadn't shown any improvement in 24 hours (unfortunately, we had to put her down before we got the chance to see how she would respond to the antibiotics the second time).
So, I think it'll be a couple days at least. If he's not eating, try to get something in him until he'll eat. Force feed him (I used a popsicle stick and kind of shoveled tiny scoops of food into Zoey's mouth...but most people use a syringe, I think) or add a bit of water to his food and see if he'll lap it up (you may have to get him started if he can't smell). What kind of food are you feeding him? If you can get some Hill's a/d from your vet, that always seems to be a hit with the sick kitties! Also, many cats will eat baby food (chicken with broth kind - make sure there is no garlic or onion).
Hopefully the antibiotics start to kick in soon! Just make sure you keep him on it for the entire round!