I got asked this question the other day and its a tricky one and I m not sure that there is any easy answer  other than using one plantsman's instinct and observation. 

Apparently our gardens here at the nursery are on an old Waikato River course and consequently the villa and office both sit on an almost sand bank position. The villa garden dries out way quicker than the office but I confess I have been watering the office garden regularly to keep it nice for our clients to visit.

At the villa the last six weeks without rain and no watering at all has seen the lawn go totally brown and the winter roses there have collapsed to, almost, the point of no return.  We haven't the water to do all the gardens and of course the nursery must come first, new planting second and established garden get emergency watering only!  I did water the hellebores for a long deep watering yesterday and it will revive them for the next couple of weeks and hopefully save the grass from dying ...  if we don't get any rain then I will have to do this again.

How much and how often to water will depend on the plant type, how long it has been established and to what level you expect the plant to perform. Old established plants like my Winter Roses did need that water yesterday as otherwise they may well have died. The fact they are on almost pure sand exacerbated the problem. In a clay, or more heavy soil that retains water, they may have lasted longer without rain.

We have done some major plantings in some of the gardens and these were all shrubs with an attached root ball (ie from a bag) and then the rain just ceased and we are tending to water these well for a few hours - just enough to get them by - say once a week to once a fortnight. If I see their foliage droop then I'm onto it and they get a good watering. 

If you are wanting roses to flower again and perform to their best level then the more water that you can provide the better otherwise they will just, kind of, hang in there.

Vege gardens will need lots of water to grow lush crops and bedding annuals will have a similar need, once well established the watering may be twice a week to weekly depending on the evapotranspiration. New seedlings will need to be kept moist until their root systems get stronger and can buffer small periods of less water.

Its all about watching and responding to the needs of your plants and garden.