Now, while some dogs are highly active in the great outdoors, most pet dogs spend their leisure time on soft grass, groomed trails, and indoor flooring, which means they don't wear down their nails on their own, and a little intervention is required on the part of homo sapien.
The reason it's important to keep your dogs nails trimmed to an appropriate length is twofold. Firstly, allowing them to become too long can lead to improper foot placement, which affects the movement of your dog up through the bones of his feet, into his legs, hips and shoulders, and, if left unchecked, can in turn lead to arthritis. Secondly, some nails will grow into a curled shape and can become embedded in the soft tissue of the pad, where they continue to grow more deeply. This is a painful condition, made worse with every step taken, added to the risk of infection. The good news is, these conditions are completely and easily preventable.
To start with, ask your groomer to suggest a nail trimming schedule for you based on your dogs activities. Your groomer will also be able to guide you in what to look for, so that you too will be able to recognize when it's the appropriate time to trim his nails. If you don't already have a groomer, ask the folks at your local dog park whom they recommend, and be sure to choose somebody experienced and reputable.
What happens if his nails are already too long? Well, in a dog whose nails have been overgrown for a long period of time, the quick - which provides blood and contains nerves - enlarges along with the nail in order to nourish it. The problem arises when the time for a nail trim does come, and the nail can only be trimmed as far back as the quick will allow. Often times, the length of the nail after trimming is still too long for good health, in which case the quick needs to be encouraged to recede. This is done by trimming the nail very close, but not into, the quick on a regular schedule of every 7 - 14 days. This procedure is best done by an experienced dog groomer, as it is quite precise.
In closing, every dog is an individual, but if you don't have a groomer, and you want to trim your dogs nails yourself, a good rule of thumb is to trim them every 4 - 6 weeks.