Because each annual has its own water needs, there's no one-size fits all rule to supplying the plants with the moisture they need.
The most drought-tolerant varieties, including lantana, California poppy, gazania, nasturtium, ptilotus, and moss rose may not need any supplemental water after they get established in your garden. Others don't hold up well to drought and require consistently moist soil.
Regardless of how often you water, keep your plants healthier by using a soaker hose. This permeable hose slowly seeps water into the ground, directly at the root zone. It keeps plant foliage dry, which helps your plants resist disease (since many diseases love wet leaves).
If your garden is blessed with rich soil or you amend it with compost or other forms of organic matter regularly, you probably won't need to feed your plants. But if you're cursed with poor soil or growing plants in pots, fertilizing can be helpful.
In most cases, all you need is a general-purpose garden fertilizer. Be sure to follow the directions on the packaging.
You might be tempted to use more fertilizer than is recommended, but you can have too much of a good thing. Overfertilization may make your plants flower less, suffer root injury, or even kill your annuals.
Because plants in pots can't reach farther into the soil to find more nutrients, they depend on you to feed them. One easy solution is to use a slow-release plant food. You just need to apply it once or twice a season and it feeds your plants for you.