As instructed from the Simple Guard 3 web page, the product can be applied as soon as the dog has dried after a bath. You do not need to wait 48 hours before or after to apply the product.
Q: Can I bathe my dog before or after I apply SimpleGuard 3?
A: Yes, your pet just needs to be completely dry before applying SimpleGuard 3 and the product needs to dry completely before bathing. You do not need to wait 48 hours before or after application. The shampoo you choose is very important, too. Do not use detergent dish soap or detergent shampoos; this will remove the SimpleGuard 3 from your pet’s skin and hair, resulting in loss of protection for your pet. Remember, if your pet is receiving therapeutic shampoos and rinses from your veterinarian for other skin diseases, they may affect the duration of action of SimpleGuard.
I think this is very brand-specific. According to the packing on the REVOLUTION product, as well as their website, you should wait 2 hours after a bath to apply the product.
REVOLUTION is quick-drying, nongreasy and waterfast, with no unpleasant odor, so you can cuddle and play with your dog as soon as the product dries. Bathing your dog 2 hours after application does not reduce the effectiveness of REVOLUTION.
I have used other products in the past that had a 3-day wait period on either side of a bath before it could be applied. It's best to read the labeling specific to your product; if that still leaves you with question, contact your vet.