I would skip the alcohol. I cannot imagine this does anything but cause pain for your baby. If you're worried about infection, use dawn soap and water to clean his wounds instead of the alcohol.
I am going to assume that you're having issues with your pet scratching until he is raw. Unfortunately, your opening question didn't mention exactly what was going on. So, this is what my response will be based on.
50mg of Diphenhydramine is a reasonable dose of benadryl for his size. Make sure that whatever preventative you're using that it's one available through your veterinarian. Not all flea/tick preventatives are created equally. For example, you'll get better efficacy from Frontline than you would from purchasing a preventative from the grocery store.
As far as "how long should I wait?", this really depends. Sometimes we learn exactly what is causing allergies...sometimes we don't. Alot of times, without doing diagnostic testing (skin scrapes, allergy testing, food trials, etc) we never learn exactly what is causing the problems. By starting on allergy medication, we may figure out what works to stop the itching...but never exactly what is causing it. I would say that if your pet is not feeling relief after 48 hours of taking benadryl every 12 hours, it's time to turn this case over to your veterinarian. There are medications in conjunction with oral antihistamines to make your pet's skin feel better so he's less likely to scratch (such as sprays and creams) and the skin can heal. You can then work on finding an oral medication that will help to stop the itching. Your vet, as I have mentioned, may talk to you about eventually doing dermal allergy testing or a food trial to see if there is an underlying issue that is something other than an inhalant allergy.
Good luck with your companion, Tyson!
Sami
