Material that has decomposed adequately will be brown and crumbly. It will have a fresh, earthy smell. The pile will also no longer heat up when you turn or mix it. When you examine the pile and it has reached this point, it can be used in the garden. There are several factors that will affect how quickly the manure will reach this point and how much you can utilize around your garden.
1. Does it contain bedding? Manure that contains sawdust/wood chips will decompose slowly because of the high carbon content of the bedding. If your horse manure includes wood chips or sawdust, consider layering the material with grass clippings (a good nitrogen source) to speed the process. Manure alone or with straw will decompose readily on its own.
2. Management. Manure that is piled and left alone will decompose slowly. This can take three to four months if conditions are ideal. It can take a year or more if the starting material contains a wide carbon:nitrogen ratio (as is the case when manure contains wood chips). If you pile the material and aerate it by sticking in pipes (see the publication at the end of this answer) and/or turning the material often, you can reduce this time to as little as eight weeks. This aerated process is commonly called composting. It has a number of advantages over stockpiling manure and allowing it to decompose unmanaged:
– You can use the product sooner.
– The heat kills most weed seeds and pathogens (making it safer to apply to vegetables).
– The end product will be more consistent.
3. Application to the garden. If it is to serve as a mulch around the flowers, it can be applied from 1 to 3 inches thick. This will suppress weed seed growth and help conserve moisture around the flowers. Compost does contain nutrients for the plants, but they will release fairly slowly. Fertilize the flowers as you normally would early in the season. Later on, you can fertilize if it appears the plants need it.
If you apply manure compost to your vegetable garden (about one pound per square foot maximum), make sure it is applied at least 60 days before you harvest the crop. You will still probably need to apply fertilizer as you normally would, especially early in the season. If you continue to apply the manure compost over a number of years, you will be able to gradually decrease your fertilizer use.
Some additional reading on horse manure composting is at: Composting Horse Manure.