The amount of water you should give tomato plants depends on the temperature and the size and amount of leaves your plants have.
Generally, the soil or compost should be just very slightly moist so that the roots have access to both water and air. Many people over-water tomato plants thinking that they're being kind, but actually too much water stunts their root growth and slows down their development.
It's best to water in the morning so that your plant doesn't have to sit in wet soil over-night when the temperatures are low. If you didn't water the plant enough, its leaves would start to wilt - if they do, give them a mist spray of water.
Perhaps the most important thing is to make sure that the entire area of the soil that the roots are in is slightly damp - dry areas will prevent access to the food the plant needs.
Hope that helps ... watering tomatoes can sometimes be a problem for even the most experienced tomato grower!