I understand that you gave your fellow Titan a Deramaxx tablet for joint pain and now he is showing signs of suffering from allergies, so you want to know how long to wait before giving Prednisone (a steroid).
Ideally there must be a 5 to 7 day washout period between different nonsteroidals or nonsteroidals and steroids to avoid causing side effects including gastrointestinal ulceration and bleeding, as well as organ stress and damage (kidney and liver disease).
Given his age, even though it was only one Deramaxx tablet, I would be pretty strict about waiting at least 5 days.
In the meantime to try to keep him comfortable you can try using a combination of antihistamines and high doses of omega-3 fatty acids. In combination fatty acids and antihistamines work synergistically, much better than either one alone.
You can try:
1)Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at 1mg to 2mg per pound of body weight or one 25mg capsule per 15 to 25 pounds of body weight orally every 8 hours.
OR 2)Claritin (loratidine) at 5mg per 25 pounds of body weight once or twice daily.
OR 3)Hydroxyzine at 1mg per pound of body weight orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
OR 5) Zyrtec (Cetirizine hydrochloride) at 1/2 mg per pound of body weight orally every 24 hours. That would be one 10mg tablet per 20 pounds of body weight. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because dogs cannot tolerate decongestants.
Some dogs do better on one antihistamine rather than another. Give the one you pick a week trial and if it isn't working try another. Be aware antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example a 128 pound dog could take 2560mgs of EPA per day.
Spot baths with cool water and an oatmeal shampoo or chlorhexiderm shampoo (which is antibacterial and antifungal) and a conditioner with an antihistamine may help. The water rinses off allergens and the cool temperature soothes itchy skin. Do not bathe your fellow for 2 to 3 days before or after applying flea control products or the bath will interfere with the product's efficacy.
Please let me know if you have any further questions.