I see that you went offline.
I am concerned about your kitty so I will give you some general recommendations.
In case he is reacting to the flea medication you need to bathe him fully in cool water and a degreasing agent such as Dawn dishwashing detergent.
Do not vigorously rub the area, as that will increase absorption. But do gently bathe that area to remove as much as possible. The less he absorbs the better. If you can still see where the medication was applied concentrate on getting that area clean. You may need to bathe him several times.
If this is a pyrethrin based product common symptoms of this toxicity are drooling, nausea, diarrhea, dilated pupils, tremors, incoordination and in very severe cases seizures.
The problem with this toxin is if they cannot eat and drink they get very dehydrated and worsen quickly.
If the tremors are very bad they can cause an increase in body temperature and muscle toxins which can cause brain damage and kidney failure.
So if their body temperature rises and they are tremoring consistently we put them on fluids to bring down the body temperature and flush out the muscle toxins.
If actual seizures occur he will need intravenous medication for that.
At home all that can be done is decontamination (bathing) and nursing care.
That includes keeping him quiet and away from stairs so he doesn't fall and hurt himself, and helping him eat and drink. Keep an eye on body temperature.(concern is a rectal temperature more than 103.5F) Keep him in a quiet area as the more he is stimulated the more he will tremor.
If he seizures he must be seen by a veterinarian.
There is nothing that you can do at home for that.
As far as how long his symptoms will last, that will depend upon whether we can get most of this off or not.
Usually symptoms last 2 to 3 days if we can get most of the product off.
If this is an extreme exposure then symptoms can last a couple weeks.
Very rarely there is long term damage.
Please let me know if you have any further questions.