The question is not how long but how much and to get that measurement you need a rain gauge. If your border is long several rain gauges spaced apart appropriately in the middle width of the border will do. Now to the VARIABLES....how much water you need depends upon the type of soil you have and the amount of drainage and percolation it provides.....as to your plants, are they new plantings or well established....as to the sprinkler method of watering, some plants take that method well others do not and will develop leaf or root problems...as to the sun, is the border in full, partial or shade?...as to the weather, how frequently does it rain? The rain gauge will tell you whether or not your border needs supplemental watering. As a general rule of thumb the average garden requires one inch of water a week and the rain gauge will give you that information quite aptly.

Thanks Loosestrife. My border is well established with a mixture of inherited pants, newly planted, annuals to fill in the gaps. My borders have a cottage garden look, a mixture of all sorts! My soil is of good quality, well draining, situated in parts in full sun, parts in partial shade, depending on time of day. I will go out and purchase a rain gauge and stick to an average of an inch of water a week. Strangely enough we haven't had any rain for over a week which in itself is quite unusual (not complaining though) so gave my garden a good soaking last night.

Great Amsterdam! When purchasing a rain gauge you might consider one of the decorative ones. I had one where a frog was holding it. Ok, now that you have replied lets go high tech. Your garden soil should be moist to a depth of six inches on average. Of course you could dig down and grab a handful of soil to find out but there is an easier way and that is an outdoor soil moisture meter (hygrometer). Instead of digging, all you have to do is stick the probe into the soil to the appropriate depth and either by gauge or lights it will tell you how the soil at that depth is doing moisture wise in any part of your garden. It is a good investment and prices start at around seven dollars- US.

Temperatures in the UK are so variable it is difficult to give blanket advice on how much per week - in warmer weather there will be more evaporation and in a cooler spell you won't need to water at all even if it hasn't rained. A rain gauge will not tell you whether the plants need watering. Daily sprinkling, at least in the UK, encourages plants to have shallow roots as they go where the water is. In our country you are better watering very thoroughly less often, to encourage deep rooting. Most established British perennials don't need watering every day and you can usually tell by looking at them which need a good soaking. If you really want to water daily though, consider a soaker hose rather than a sprinkler, so the water is all delivered to the soil not on the leaves, where not all of it will reach to ground. If you really worry about soil moisture you can buy a little water metre which has prongs that you push into the ground, and the moisture level can be read off on the metre.

During hotter, dry spells, wait a maximum of 7 days, sometimes 6 if you spot wilting on smaller perennials, then put the sprinkler on for a minimum of half an hour, but preferably an hour (depends on your water pressure) in each patch. Absolutely the best way to water a garden.

Note that mature shrubs can go a lot longer without water, but perennials are a good guide to how dry the soil is - the small ones show signs of wilting and stress before anything else does, so that's the time to get the sprinkler out and give it all a good soak.

Not sure that you really need to buy rain gauges, moisture meters etc - just use a thin wooden stick (like the ones stuck into potted plants) to push into the soil - like testing a cake - if it comes out dry you need to water, if it comes out with damp soil on it then you're ok.
Or, as Bamboo says, look to see whether anything is wilting.

I seem to be getting everything wrong today. Will crawl away and shut up !

Hey! Wait For Me!!!

Steragram, what you on about! You haven't got anything wrong, my answer sort of echoes yours, except for the leaky pipe part - leaky pipe is okay in areas you don't want to cultivate frequently, but it drives me bonkers where that's not the case - I always end up digging it up by mistake or putting a fork tine through it, which is why I didn't suggest it.

Thank you so much Bamboo - have been brooding on this one! As we are all supposed to be conserving water I thought just putting on as needed was better than just broadcasting it everywhere

Lol Loosetrife-
Do try to keep up!.

It's very different in the States, this watering business, but a lot of areas there just don't get frequent showers like Europe does.

Yes, originally I started by saying that, but looked up the climate of NJ and the total rainfall seemed similar to here so i deleted it.. I've never used a soaker hose but know a lot of people swear by them. I still carry cans as and when needed...

Really do need a hose for the veg plot if this weather continues though. We do have one but it isn't long enough. But I will walk round with it not sprinkle.