How long it takes dog poop to decompose depends on your dog's diet and the climate. In colder regions, that fecal material might be around for as long as a year before natural decomposition occurs. That doesn't mean you can't rush Mother Nature by composting dog waste. It's good for the environment and helps fertilize nonedible plants.
Dogs are omnivores, meaning they eat animal and plant material. If your dog primarily consumes meat, his feces take longer to decompose than that of a canine eating food filled with grains. However, choosing your dog's diet based on fecal decomposition isn't necessarily good for his health. Ask your vet for recommendations for the best food for your pet based on his individual needs.
As a conscientious dog owner, you always pick up your dog's poop when you're out for walk or clean up after him in the yard. From an environmental standpoint, using nonbiodegradable plastic bags to collect the waste and then throwing it in the garbage isn't helping the problem. Purchase flushable poop bags for collection and flush your dog's poop down the toilet. That's a more environmentally friendly approach than letting it spend a year decomposing -- or even mummifying -- in a landfill.
You can scoop the poop into a digester, which breaks down the matter more quickly with the addition of special enzymes and water. Since you're regularly adding feces, water and enzymes, it can be hard to tell exactly how long it takes the original "load" to decompose. However, in warm weather that should take between two and three months. One caveat: Don't install a digester in clay soils. The liquified waste either can't leech back into the earth or takes a very long time to do so. You also can't use this method when the ground is frozen, as the poop just freezes and won't start decomposing until the weather turns warmer.
You can compost dog poop along with other organic material in your compost bin or pile. Turning the decomposing matter regularly helps the matter break down. The US Department of Agriculture recommends adding one shovelful of sawdust or some other carbon source for every two shovelfuls of dog poop. The USDA estimates that it takes between six to eight weeks for the compost to "cook," breaking down sufficiently for use. While proper composting should destroy the pathogens found in a meat-eaters diet, it's not recommended for use in vegetable gardens or around fruit trees. It's fine for use in flower beds, potted plants and as landscaping mulch.
If you're interested in vermiculture, or worm farming, dog poop can help feed your worms just as well as the food scraps and other organic material you add to the worm bins. Mixing dog excrement and vegetable waste means you must add leaves or newspaper to the bin, as they provide carbon. Because the worms eat the poop, it actually doesn't decompose. A pound of red worms can eat as much as a half-pound of waste each day. Worms produce their own weight in waste daily, which you can use as fertilizer. As with compost, don't use dog waste fertilizer on edible plants.