I've read hundreds of times to use "well rotted" manure, but none of the literature specifies what length of time this is. We have a horse stable, thus LOTS of manure, but I never know how long to let it rot before using it on my lawn, vegetables, flowers and shrubs.
Let manure age for three to four months before adding it to existing plantings or soon-to-be-planted gardens. Fresh manure can be dug into annual flower and vegetable gardens in the fall. The excessive levels of nitrogen and soluble salts that are harmful to plants will dissipate by spring planting. Or create a compost pile using manure and a little soil to help convert green plant debris into a valuable soil amendment. Use some of your fresh manure to brew your own manure tea. Place a shovelful or two of fresh manure into a burlap or mesh bag. Set the bag in a large bucket filled with water. The nutrients leach out of the manure into the water. Use the manure tea to fertilize your garden plants.