If you have newly planted seeds you will want to water often. For germination to take place, moisture is important. For watering seeds, you will want to water them lightly but consistently.
After the plants have started growing, you should water once or twice a week. Water plants deeply so that their roots are encouraged to grow deeper. Shallow roots on the plants make them dependent upon more frequent waterings. Water them long enough that about 6 inches of soil is saturated.
During extremely hot weather and droughts your plants will need to be watered a bit more frequently as they will start to suffer and show signs of wilt.
To keep your water from evaporating too quickly you will want to water the plants in the morning or evening, preferable before 10 a.m. or after 5p.m. (the later the better, especially on particularly hot days.)