Well, the simple answer to this question is: how often do you want to bathe your dog? Unless a dog has skin problems, there is no specific need to bathe the dog except to make him a more enjoyable companion.
Just like people, some dogs get stinky faster than others. I have known dogs that have never had a bath in their life and do not have an unpleasant smell.
The SPCA says dogs need to be bathed about every 3 months. If your dog gets dirtier, for example, by romping outdoors, you can consider lathering up more often.
What should you use to bathe them?
You can bathe a dog with normal skin with a mild hypoallergenic dog shampoo. Mild Aloe and Oatmeal Shampoos normally work well without skin irritation.
Shampoos made for people aren't toxic, but they may contain fragrances and other ingredients that irritate pets' skin. Human products work best on human skin and veterinary products are designed to work best on dog skin. The chemistry of a dog’s skin and fur are different than the chemistry of a human’s skin and hair.
While dish soap or your favorite human shampoo might strip away the dirt, and more importantly the odor from your pet’s coat, it will also strip natural oils from their fur and may irritate their skin. Dish soap will also strip away topical flea and tick products from your dog.
If you want to bathe more often than once a month, use a soap-free or moisturizing shampoo to prevent the skin from becoming dry.
Consider brushing instead!
I do think it helps to brush them. Routine brushing enhances your dog's appearance by distributing natural oils throughout the entire coat. It makes the dog’s coat look nice, healthy, and glossy. It also prevents hair from knotting or clumping and whisks away dirt, burs, and other outdoor debris. If your dog stays fairly clean with regular brushing, you might get away with fewer baths.
Plus, there's a big emotional payoff. The brushing actually helps you develop a good bond with your dog.
How extensively you need to brush and comb depends on your dog's coat. Long-haired breeds, like golden retrievers and Newfoundlands, will need longer, more intense brushing almost daily. Short-haired dogs, like Dalmatians or beagles, aren't as hard to brush, but regular sessions will still cut down on shedding. Use a steel-tooth comb to remove tangles and then a stiff bristle brush to get rid of loose hair.
Though frequent brushing may do wonders for your dog, the same is not true of baths. Don't overdo it. Most people bathe their dog more often than they need to. Too many baths will strip the coat of natural oils that protect the skin, and your dog's coat will lose some of its shine and luster. However, there are dogs that will need more frequent, medicated baths, but only if your vet recommends it.
A final insight pertaining to bathing your pet is to comb their coat prior to bathing. Wet fur mats more than dry fur so a wet tangled coat is harder to brush out and will take longer to dry. This small detail can save you time and prevent an uncomfortable brushing for your pet.
After a bath, your dog will smell good, look good, and probably feel good. Make sure your dog is dry before you allow it back outside or it will feel good enough to dry itself. It will streak from the tub straight outside to find a new exotic aroma to frolic in and bring home to share.
What about cats?
Try to skip baths if you have a cat. Cats don't really need baths as long as they're able to groom themselves. Occasionally cats do require bathing. If your cat's coat feels oily, greasy, or sticky, a bath is in order. Also consider a bath if your cat has gotten something in their coat like motor oil that could be harmful if ingested while grooming.
First, give a thorough brushing to remove loose hair and mats. Then bathe your cat in lukewarm water with mild cat shampoo and dry them with a towel. Keep them in a warm place until they fully dry to prevent lowering their body temperature.
To cut down on hairballs:
Cats and cat owners alike detest hairballs, and for good reason. It's great that felines love to lick themselves clean, but they can swallow a lot of hair that collects in the stomach. The hair buildup in the stomach causes hairballs.
To prevent hairballs, brush your cat regularly. Your cat won't take in as much hair, especially if you wipe her with a clean cloth after brushing to pick up any loose hairs. If your cat has long hair, try to brush every day. Limit brushing sessions to 10 to 15 minutes. Longer sessions might upset your cat.