Hello Terry,
A rule of thumb MSU Extension uses for all plants and vegetables is 1" of water each week. In drought conditions, that amount may be 1/2" more. This also depends on where your tomatoes are planted. If they are in-ground and the soil drains well, an inch of water a week is adequate. You also need to include any precipitation from rainfall in the total. Tomatoes grown in pots have different water requirements. Since you didn't indicate pot-grown tomatoes, I assume your plants are in-ground.

The following is an excerpt from an article by Iowa State Extension about growing tomatoes. 

Water

Like most vegetables, tomatoes perform best when they receive one inch of water per week. Supplemental watering is best done in the morning and applied directly to the soil surrounding the plants. Avoid overhead irrigation as it splashes soil, helping to spread common diseases like septoria leaf spot.

Soil type does not affect the total amount of water needed but does dictate watering frequency. Sandy soils require more frequent water applications, but less water applied per application.

Consistent watering is essential, especially during fruit development. Plants that do not grow with even soil moisture can develop cracks and blossom end rot.

https://yardandgarden.extension.iastate.edu/how-to/growing-tomatoes-home-garden

Overwatering and underwatering are two of the most common reasons for plant problems. I have found that using a rain gauge and a moisture meter can help prevent overwatering in the vegetable garden. Unfortunately, we cannot control Mother Nature, so we have to use good gardening practices to produce a good crop.

See the following excellent article from the University of Minnesota Extension that explains the major diseases and pests of tomatoes and how to treat them.
https://extension.umn.edu/plant-diseases/insects-and-diseases-tomatoes

I hope this information helps you with growing tomatoes, but if you need further information or have questions, don't hesitate to contact us again.

Have a great summer!