In places where natural woodlands once stood, roses will generally do well without supplemental water once established. In places that two millennia ago were wide open grassy spaces, supplemental water might be helpful even for established roses.
One very good method of irrigation is underground or drip irrigation with a soaker hose. These hoses are about 50ft long and are constructed from recycled car tires. They are full of tiny holes which let water out. They are ideal for watering all kinds of plants, and can be buried just below the surface so as to be virtually invisible.
If one has a lot of time or has a gardener, hand watering can work well. It has the advantage of being able to adjust watering amount as needed, and it makes careful regulare plant inspection seemless.
In places where powdery mildew is a problem, overhead watering in the morning is sometimes advocated.
The rule of thumb is that roses need an inch of water per week. That's when the temperature is 75F.
In places where summers are cool, roses will have somewhat lower water requirements. And in the blazing heat of the Great Plains, two inches per week may be a better number, especially for large plants. In places where roses go dormant during the summer, this also mitigates their water consumtion considerably.
Roses need water. Without it they will die. Budded roses fail frequently to thrive in a new environment because of lack of moisture; their tender feeder roots have been destroyed by the transplanting process. Roses purchased in pots might fair better except that they are smaller to begin with, and rough treatment can sheer of the finest feeder roots. Therefore, careful watering during the first season, especially during the first ten to sixteen weeks is absolutely crucial to the success of a newly transplanted rose.
Roses, fortunately, are deep-rooted plants. Deep-rooted plants have the capacity to extract water from the sub-soil long after the surface has become dry. Finally, many hybrid teas and teas have relatively few leaves. All of these factors combine to make most rose cultivars fairly tolerant of moderate dry spells once they are firmly established.
This said, a rose will be at its best if it is well watered. It will bloom well and it will be more resistant to disease. Leafy cultivars such as Albas and Rugosas my require more regular watering than more sparsely leafed types. And it can take some time to become well established.
During the first year after transplant it is imperative that the roses receive an inch of water each week, either from natural or artificial sources. During the first three months water daily. After the first three months, water at least twice a week. Established roses will need watering less frequently, but still an inch of water per week is roughly what a rose will require.