Ask Mr. Smarty Plants

That is a really broad question, and not one for which we can provide an exact formula. Since you are living in Austin, we'll try to cover watering in a dry environment. And since you are asking about it now, we will assume you are putting your new landscape in during the Fall, which is usually the best time in the part of Texas. Start out, just for general background, by reading this article from Washington State University Extension on Watering Home Gardens and Landscape Plants. Although it is dealing specifically with a different part of the country, the general principles still apply.

This has been an incredibly dry year in Austin, and everyone is being encouraged to conserve water. Even plants that are drought tolerant are going to need some watering as they get established. And succulents can go longer than other plants without water, but not water-free. The best thing you can do to help your plants survive without having to pour water on them every day is to prepare the soil that they will live in before they are ever planted there. Living in Austin, you probably have an alkaline and perhaps rocky soil. Digging in compost, leaf mold or other organic matter will help to amend the texture of the soil, permitting it to drain better. If you run water on a patch of your garden until water stands there, and it stands without draining for more than about thirty minutes, you probably have a poorly-draining clay soil. You can't replace that, but you can sure help its draining quality with the method suggested above. Once you have your plants in the ground, put down a layer of shredded hardwood mulch. This will help (a little) with weed control, but mostly it is for moisture control. It keeps the roots warm in winter and cool in summer, and helps keep evaporation into the dry hot air  to a minimum. As the mulch decomposes, it will continue to amend the soil, and you can add more mulch.

Once the plants are in (and if they are already in, have you mulched?), different plants are going to need different kinds of attention. If you have a sprinkler system, you are probably relying on overhead watering. Do not do this late in the evening, as moisture staying on leaves for a long period can cause molds and mildew, and attract plant-killer fungus diseases. And watering in the middle of the day, especially in summer (which seems to last 6 months around here) will cause you to lose a great deal of water to evaporation before it ever gets to the plants.

Don't just set an automatic sprinkler system and forget it. In the first place, if we're getting rain, the water that's pouring out of your system is not only wasted but could cause damage from too much dampening. If some portion of your system is malfunctioning, you could discover wilted and dying plants when it's too late to save them. How much or how long to water is really a trial and error procedure. If the soil has been amended, getting moisture down about 6" for perennials and annuals is pretty good. You can determine this by digging a trowel down about as deep as you think the roots are growing, and see if there is moist soil there. 

Shrubs and trees, especially when they are newly-planted, are going to need deeper watering, especially until the roots are well-established. There should be loose soil around the bases of the trees and shrubs. Stick a hose down into that soil, and let it drip slowly until water appears on the surface. Again, if it stands too long, you need to do something to amend the soil so the roots don't drown. If you have planted in the Fall or Winter, this can be done just a couple times a week. If you planted in the Spring or Summer, you will need to do this more like every other day. This is slow and tedious, but you have a pretty big investment in those plants, you need to preserve it.

DO NOT OVERWATER! Don't assume that if a little water is good, a lot of water will be wonderful. In the first place, your water bill will be astronomical. In the second place, you will be drawing precious water from our aquifer, and returning it in the form of runoff, perhaps with fertilizer or even herbicides mixed in. In the third place, as we said above, over-watering can damage your plants. Planting natives that are adapted to the area, needing less fertilizer, water and maintenance is one of the best moves you can make.  From our How-To Articles, read these articles on Using Native Plants, A Guide to Native Plants and Native Lawns.