It's such an open question that it's almost impossible to answer on an individual basis. Water when they need it would be my advice. Check the soil and see if it's damp/moist or dry. if it's dry, give it a water. It will also depend on what plants you're growing....some shrubs such as Cistus like dry soil whilst some ericaceous plants such as Azaleas hate being dried out.
the same would go for bedding plants too...geraniums will love it hot and dry whereas lobelia will not tolerate being dried out.

Allow the plants to dry out inbetween watering a little....not to the point where wilting. Expect to be watering everyday if the weather is warm, dry and windy, don't water at all if there's been some heavy rainfall.

Hope this helps a bit, you will eventually get to know how much water to give them by watching how things grow and behave especially in different weather conditions.

Agree with everything said by Andy, with one addition - check your pots when the weather is wet too, during spring and summer and early autumn. If the foliage is bushy and the plant is large, chances are rain won't get into the pot.

If you are worried you can buy a meter which will measure pH, light and moisture. It has prongs you thrust into the soil and you can read off the moisture content on a scale. its very useful for larger containers in particular as the top often looks really dry when its quite moist underneath. I was very surprised first time I tried it.

With smaller plastic containers lift them up regularly to get used to how heavy they are when wet and when dry - a very easy and quick test.

Also pay close attention to how the leaves look - they do look subtly different if they are a bit too dry on many varieties.