The water requirements are related to the size of the plant, the ambient
temperature (and wind) and the amount of soil the plant is growing in.
The soil has to provide enough water storage to keep the plant hydrated.

As the plant grows, the water requirements increase (more leaves = more
transpiration). If you put your tomatoes in a larger container you may
be able to water them only twice a week. Adjust the size of the
container to the amount of time you have to spend on your plants. If you
have only once a week, plant them in the ground instead of a pot. In the
ground the watering requirements depend largely on the soil type, the
depth to groundwater, and the age of the plant (from transplanting:
newly transplanted tomatoes have shallow roots so they will need more
frequent watering until the roots get down to a more reliable water source).

When you water a pot, give it a bit of water to wet the soil, then wait
a minute and give it some more, then wait again and give it some more.
If you just dump it on all at once the water doesn't have time to
thoroughly wet the soil in the pot before it starts to run off. In
general, I leave a bit of space between the soil level and the top of
the pot (an inch or two depending on the size of the pot). I then fill
that space with water. When it soaks in, I fill it again. For large pots
or pots with a small width to depth ratio I do it a third time also.

Don't do this every day unless your plant really dries out quickly.
Roots need air as well as water. Unless you're growing wetland plants
you don't want to drown the root system.



Mark Anderson wrote:
...Twice a week may actually be too little water. Tomatoes tend to really
eat water in pots and when it gets hot here in Chicago, like it is now, I
have to water them every day. Some people suggest twice a day but I'm
not that diligent. My tomatoes are rather small, about 1 1/2 feet high,
so they're not drying out the pot as fast but I still check the soil and
sprinkle them every day if needed. Mine certainly can't go more than
2 days without water even at their current size. When they get big and
the root ball grows, if I didn't water them every day the stupid plants
started to wilt. Growing them in the ground is far more forgiving wrt
watering.