I think it depends on the dog and the sort of walking they do and what they walk on. Our labrador never seems to need his nails clipping, they must wear down on the rocky paths just enough. Della, the mastiff, had problems with her nails splitting. They either grow faster or, because they are narrower they are not as strong and break rather than wear. We ended up buying a small pair of nail clippers for her and just do a nail or two when they seem to be getting a bit long.
It might have been these ones from
wahl which are not very expensive. They have a guard which you can leave in place so as to not take off too much and damage the quick of he nails. They've lasted well - at least 3 years so far.
About once a month sounds reasonable, but as we do individual nails rather than the whole paw, I'm not really sure how often we cut them.