Stick your finger into the pot 1" and if it's dry water.

There may not be an easy answer to your question. Watering plants in containers depends on many things. Size of container is important, as well as type of container. I have lots of herbs in containers, and have also over the years grown many vegetables in containers. I have done this in regular containers with drainage holes, and self watering containers as well as earthboxes. Depending on the weather, having the right potting mix for containers, the specific plant as well as maturity of the plant in the container, I have added water to the self watering containers every day or two early in the season, and often twice a day as the plants mature and weather heats up. If the container has drainage holes, and has a good quality potting mix that does not become waterlogged, in my opinion, it is safe to water them to the point of run off daily. Mature vegetables in non self watering containers may need water twice daily. If your containers are not large enough to handle the vegetable in them, tomatoes for example, it can be almost impossible to keep them properly hydrated. Plants sometimes look "sad" during the heat of the day with direct sun, they droop their leaves some naturally to reduce exposure to the hot sun and reduce loss of water. When the sun gets off them, and they perk up, this again in my experience, means they are getting enough water. If they are still droopy early in the morning, they are not getting enough water. Hope this helps some, there really is no one rule fits all in this area.

What opal said. I have veggies in smart pots, so I can use a heavier mix. Still, I usually water every day unless it has rained.