It should be noted here that phosphorus does not control flowering and fertilizers containing high percentages of phosphorus should be avoided. Phosphorus should only be added if it is deficient in the soil. You should also not use fertilizers high in nitrogen, like those used for grass. Too much nitrogen will result in lots of new stems and leaves but very few flowers. In the picture below, the flowers are obscured from the flush of green leaves produced by a high application of nitrogen.
Applying small amounts of fertilizer every week is ideal. If you can't do it weekly then fertilize every two weeks. If that won't fit in your schedule then you should consider a slow release fertilizer. The most commonly sold slow release fertilizers are labeled as 3-4 month formulations. That's true if you garden in a cold climate. The rate of release and therefore how long they last is controlled by soil temperature and moisture. Most slow release fertilizers can be counted on to feed our flowers in the Midwest for about 2 months. We should apply them at planting in May, again about the 4th of July (memory aid) and again in late August. Read the package label for guidelines on how much to apply at any one application. Remember a little bit applied often is ideal.
Organic fertilizers also work well on annual flowers. They are not very predictable as to when their nutrients will be available to the plant. The nutrients are tied up in organic compounds that need to be broken down in the soil before they are available for plant growth. Organic fertilizers need to be applied well in advance of the time they are needed to fuel plant growth and should be applied several times during the growing season. In addition to feeding plant growth and flowering they also aid in developing quality soil. Dried blood meal, fish emulsion, and products made from chicken manure, sewage sludge, soybean meal and ground alfalfa are all valuable sources of organic nitrogen. Avoid using bone meal or other materials that may contain primarily phosphorus.