There are so many different view points on this the mind does actually boggle. But in my opinion there is no right or wrong - just what is right for you. Lets be honest, in the majority of cases we don't bath our dogs because it is in their interests. We bath them because it is in ours. Maybe they are just generally a bit smelly, they have rolled in something, they are muddy or have food stuck around their beards? None of that will do them any harm - we just don't like it. There are occasions when a bath is necessary - ie Alabama Rot season means it is on our dog's interests to wash the mud from their paws. When our dogs go to the groomers it is nigh on impossible to get a decent cut if they are not bathed and brushed through first, some dogs may suffer skin conditions which require a medicated shampoo, but on a day to day basis the majority of baths are not a necessity for the dogs health.
Some dogs happily go through life never being bathed, to no ill effect and their owners are clearly happy with that.
I personally find a bath several times a year with paw washes and spot washes where necessary are sufficient, but my dogs do not go on our furniture or upstairs on the beds. If they go in the sea a lot in the summer I will pop them in the river for a rinse off but not necessarily bath them fully. If I am honest it is down to lack of time and possibly a little laziness too. I do love the freshly bathed look and smell and if I could get someone to do it for me I would happily have them bathed more frequently.
How much is too much? I would suggest that if you have your dog on the furniture and in your bed and want to bath your dog frequently you can, even weekly is not unheard of and many dogs will fall in to the category, but, and it is a big but, the products you use will matter in this scenario. The more you bath your dog the more you will strip the natural oils from his coat, you can really help avoid this by using a natural, gentle shampoo (or one of my award winning soap bars which are about as natural as you will find) The saying you get what you pay for really does apply in this scenario. If you buy a bottle of shampoo for 99p it is going to be full of harsh, cheap detergents many of which you will also find in house cleaning products.
If you buy a good quality, dog shampoo it will be gentle and conditioning and will not dry out the skin. Our liquid shampoo with aloe vera is formulated from plant based ingredients and really does clean beautifully. Even better, it can be diluted up to 18:1 with warm water making it a cost effective alternative to some others.
The bottom line is, do not stress over the subject too much. Just bath as and when required depending on your life style and consider the product you are using rather than the frequency you are bathing.