Capillary mats work by keeping everything evenly moist, which means they are somewhat based on the surface area of the pot and the type of media you have. The larger the surface area that is exposed, the higher the evaporation rate will be.
Plants take up different amounts at different times.
A young plant (tomato) takes up less water than an older plant because it has a smaller root volume.
Plants in pots dry out faster than plants in the ground, because the roots are confined to the pot (unless they escape) so cannot go deeper or spread out to find water reserves in the ground.
The type and size of pot, root volume, type of media, weather (hot windy days will cause greater amounts of water lost to transpiration), and even whether the plant is growing or fruiting will impact how much water a plant needs.
I grow tomatoes in 18 gallon tubs(home made self watering container). It has about 5 gallons of potting mix (peatlite + fertilizer with micros, and a 5 gallon reservoir for water. Mature tomato plants with fruit in mid summer can drink up to 4 gallons of water a day.
Self watering containers get water similar to the way a capillary mat works. There are "legs" filled with soil that reach down into the water reservoir and wicks the water up to the soil. There is also a panel that separates the soil in the top of the container from the water below. A drain hole or two drilled about a 1/4 inch below the panel creates an air gap between the water and soil. Water is pulled up by the capillary action of the soil in the legs into the container above.