The answer to your question depends on the formulation of fertilizer you decide to use. Water-soluble fertilizers are intended for frequent application as you water. Follow the recommendations on the product label for best results. Slow-release fertilizers are designed to gradually break down and supply fertilizer over several months.
For northern regions, one application per growing season will usually suffice. In southern areas with longer growing seasons, a second application may be needed. The third formulation of fertilizer is granules. It's a good idea to mix some granular fertilizer into the soil at planting time. To keep annuals blooming well, reapply granular fertilizer every 6 weeks through the growing season. Use one pound of 5-10-5 or 5-10-10 or similar analysis for 100 square feet of bed area.