It does not help much to use the watering can little but often, instead it is better to water less often but more thoroughly so that the water seeps in and the roots grow deep. You should apply 10 - 30 litres per square metre. Drive a spade into the ground and pull it back slightly. Then you will be able to see how far the water was able to penetrate into the ground.
The earlier in the day that you apply the water in summer, the better it is for lawns, beds and plants. Because the water soaks in well early in the morning and can be taken up quickly by the roots. But never water at midday, because most of the liquid evaporates in the heat before it can soak into the ground. Furthermore, droplets on the leaves can act as a magnifying glass for the sun. The evening is not so ideal, because the leaves stay wet overnight and this invites fungal infections and mildew. It also attracts snails.
The answer is: it varies. Of course you need to water more in sunny areas than in the shady regions of your garden. Also, some plants are especially thirsty, such as fuchsia. But in general it is possible to say that the amount of water to be applied increases with the size of the leaves. Seedlings and newly planted items require water every day for a while.
By the way: Only water just before the leaves start to droop on your plants. In this way you force them to form long roots so that they can fetch moisture deeper from the earth as well. The plants will become more robust as a result.
You do the same with a lawn: In a drought it is best to soak the area once or twice a week, right down to the roots. We recommend around 10 to 15 litres of water per square metre lawn. Grass under trees also needs more water because the trees take it as well.
You do the same with your lawn. You only need to water it two or three times a week. But apply a lot of water so that the water can seep down at least 15 cm. Since you cannot dig up your lawn to check this, up to 15 l/m² every three days should be enough for loose sandy soils. If in a somewhat heavier loamy soil you need to keep the tap turned on longer, up to 20 Liter/m² need to be applied here.
You can of course delegate the work of watering – to mechanical and automated systems.
Oscillating sprinklers are ideal to regularly water large areas. Simply connect the hose, turn on the tap and the water is distributed evenly over the area. The same applies to rotating sprinklers, which are mostly used on lawns. They wet over a full circle, but can also be set to water only an arc, depending on the model and the setting. By the way: If you garden hose is too short or your garden is too long, then use a WOLF-Garten hose reel trolley. That already gives you several more metres.
Computer-controlled watering and irrigation systems have also found their way into private gardens. No wonder, they can be programmed beforehand and then work all by themselves. A fully automated system of this kind is admittedly extremely convenient and reliable, but the convenience comes at a price. It is your decision as to whether or not such an investment is really worth it.
The oldest watering system of all still has its place. With a watering can in your hand you can decide for yourself which parts of your garden you will water, and how much. You can use the rose to direct somewhat less water evenly and gently onto small and delicate plants. Or take off the rose and water with a hard jet and a lot of liquid at once.
However, the watering can has another advantage - and even when compared with watering systems: With bushes and shrubs and vegetables you can specifically wet only the soil and so prevent the water from getting the leaves and forming droplets.