Firstly I need to mention drainage, always make sure that any plants that are in pots and containers have adequate drainage for excessive water to drain away from the plants roots, as much as plants require water they also do not like their roots immersed in water for long periods of time( excluding bog and pond plants) as this will lead to their tiny feeder roots rotting which in turn can lead to eventual death of the plant itself.

There are many factors to take into account regarding how much water to give a plant:

Size of the plant, size of the container, type of compost, the weather, time of year and also the position of the container.

With all these different variations it is impossible to ever give an exact answer, so we need to look at a general solution to container watering that can be adapted for whatever situation your plants fall into.

My first piece of advice is to use a watering can, if possible, rather than a hose as it is a lot easier to gage the amount of water you give your containers. Watering with a hose can be quite deceiving and you may be surprised how little you are actually give the pots!

It is better to give your plants a good soaking and then allow them to dry out between waterings as this will produce plants that are stronger and healthier that will also cope better with the intense heat of the summer, as the plants will send out more roots in search of moisture which in turn will then absorb more water when then are next irrigated.

So, with this in mind I suggest you water your containers three times a week e.g., Monday, Wednesday and Friday.

So how much water is enough? 

You need to give enough water that saturates the compost in the pot and you visibly see the water draining from the holes in the container, once you start this routine you will notice that during your watering week you will need less water on the second and third waterings of the week, depending on what the container holds, for example a small tree growing in a container will have a larger root system throughout the pot to absorb the water rather than bedding plants in the same sort of container where the roots will only be a fraction of the depth into the pot hence requiring less water as the compost in the bottom of the container will hold water for longer.

As mentioned earlier after a few waterings with a watering can you will get used to how much each container requires and you can easily adjust as necessary giving a little more or less water depending of the conditions such as the weather.