Is an automatic garden irrigation system without a cistern worthwhile?

20-30 minutes is never enough. Just think about how much flow rate you have and how high the water demand of the plants is… It is rather subjective.

We are only allowed to screw a calibrated water meter onto the outside water tap and then we have to photograph it and replace it after the calibration period expires. We cannot install a water meter inside because the water is only being passed through (that's what we were told). If we really laid a hose through the garden, then we would also have to move the curb that borders the drainage stones lying around the house because of that.

With the right planting, you hardly have to water flower and perennial beds at all. I do it like Winniefred. Planting must be able to cope with drought. Watering is done when necessary. Surprisingly little is needed for properly deep-rooted plants. Roses, for example, easily manage 4-5 weeks without rainfall in full sun. Deep roots are not achieved by drip irrigation or moistening a little daily with the garden hose.

Brown lawn is normal in summer. It is considered good form here. No one in our area has an irrigation system. Dry grass endures a lot, as I see with ours too. It takes a long time to turn brown, and it recovers quite quickly. However, it grows relatively little during dry periods, and that is what the weeds love. You cannot get a soft, evergreen lawn this way.

I am really surprised by the (alleged) water requirements for gardens expressed here.

you save yourself hours of watering the beds every day.

I think for the daily watering

No garden needs to be watered daily. Exceptions are freshly planted or sown things during dry spells with heat on sandy heath soil.
Or you really paid no attention at all when choosing the plants except for their appearance.

So you can also make life hard for yourself. I haven’t watered my tomatoes at all since planting them two weeks ago except for the initial thorough watering. And they are under a roof, so they don’t get any rain. Of course, I mulch the bed. And tomatoes are plants with a rather high water requirement. When the leaves eventually start to droop, it gets a good watering again.

We only watered our perennial bed in the first year, when everything was new and not well rooted yet. The plants were selected according to heat and drought tolerance. Since then, watering: none. What survived continues to live like that and looks good. And weeds hardly stand a chance that way, only dandelions and hibiscus seedlings from the neighbor’s bush are really tough.

But in the first year, the stuff needs water, water, water. And preferably only every few days, but quickly and heavily, so the plant roots as deep as possible. This also applies to lawns.
And later, too. No one can tell me that they let their painstakingly created garden completely die off if there is a drought summer like 2021 or 2022 and then pull out the hedge the next year and start over from scratch.

This applies for the start in the first year. But bushes, i.e. usually deep-rooting plants, should cope on their own after settling in. Otherwise, you have the wrong species in the wrong location.

What really needs to be watered regularly are pots, when it’s really warm and dry.

Well, that's great. Then buy it at the hardware store, depending on the sewage fees you'll have covered the costs again after 15 to 20 m3. And you'll easily need that amount within 5 years.

I am surprised by the (alleged) water needs of gardens sometimes expressed here.

No garden needs to be watered daily. Exceptions are newly planted or sown things during dry phases with heat on heath sand.
Or you have really paid no attention at all to anything except appearance when choosing the plants.

So you can make life difficult for yourself. I have not watered my tomatoes at all since planting them two weeks ago except for the initial thorough watering. And they are under cover, so they do not get any rain. Of course, I mulch the bed. And tomatoes are plants with rather high water needs. When the leaves eventually wilt, I water again with a proper watering can.

We only watered our perennial bed in the first year when everything was new and not well-rooted yet. Plants were selected based on heat and drought tolerance. Since then, watering: none at all. What survived continues to live and looks good. And weeds hardly manage to grow, only dandelions and hibiscus seedlings from the neighbor’s bush are really tough.

This applies for the start in the first year. But bushes, i.e., usually deep-rooted plants, should manage on their own after establishment. Otherwise, you have the wrong kind at the wrong location.

What really must be watered regularly are pots when it is really warm and dry.

But that all depends on the soil and the location. At my father’s place in Potsdam, where he has great soil, we didn’t need to water at all this year. With our sandy soil, I could at most plant cacti. Neither roses, nor perennials, nor shrubs would survive more than a few days.
So you can’t generalize this.