How much and when to watering varies on a number of factors, such as weather variations, your soil composition and how high the water table is in your area, and whether you are mulching the ground or using other water-saving techniques.
Generally the advice for watering most vegetables is the same:
Water regularly when they are seedlings as they have less established root systems and need to have sufficient water (without over-watering) to grow consistently without being held back by dry soil.
Water less frequently but more deeply as the plants grow on - only when the soil feels dry on and just below the surface. This encourages the plants to put roots down deeper to where they can access soil moisture more easily. Plants in containers need more frequent watering as they dry out much faster.
Use drip irrigation if you need to provide more constant supply, particularly in areas where water evaporates due to sunny hot days.
Some plants have special requirements and these are noted in the Plant Information boxes in the Garden Planner (e.g. Celery requires more constant moisture and moisture-retentive soil than other vegetables and this is mentioned in the Notes section).
Usually it's best to water the soil rather than the leaves to make sure the water reaches the roots where it's needed, and also because damp foliage can encourage fungal diseases such as potato blight.