Much of the watering need depends on your climate and soil.
Here in Houston, we have the "arm pit" of humidity of the US. (Sorry for the graphic depiction) and heavy clay "gumbo" soil.
In the first year of growth, grapes tend to like an evenly moist and loose soil. Yet they will not tolerate "wet feet" for prolonged periods of time.
I dug a pit 3 times a wide and 3x deep as the root ball and replaced the soil.
This helped the vines for drainage and such in the first couple of years.

I also utilize drip irrigation for my grapes.
The first year I started the vines, I watered 4 gallons every 3 or 4 days, and liquid fertilizer every 14 days.
Grapes tend to establish a deep root structure in the first couple of years. Established grape plants are pretty drought resistant. If you water it at all, (which I do) give it about 25 (clay) to 50 (sand) gallons thru a 2gph (clay) to 5gph (sand) dripper every one and a half to two weeks once established.

I ONLY water in the early morning. This is due to the many fungal disease problems which many grape varieties are susceptible to. With the warm ground temperatures and high humidity here, I do not dare to tempt fungal problems with afternoon or evening watering. I have found my best friend in the early morning is a good quality electronic water timer. :-)

After the first year, if possible, avoid watering grapes after mid August as watering will greatly delay fruit ripening.

I also spray for fungal problems. For myself, I have found that a general spray containing Bacillis Subtillis works well. This is normally available at quality garden centers and on the web. One product readily available which contains Bacillis Subtillis is Serenade garden disease control concentrate.

Hope this helps, and keep in mind the zone, soil, and climate differences.

The Fruitnut