The amount of water required varies tremendously, but on average we us 35mm per week for normal flowers up to 56mm per week for bushes.
You can water flower beds by:
Pop-up sprinklers. In very formal gardens pop-up sprinklers are used because when they are not in use they are out of view. However, they are expensive to install and move as your planting develops. Pop-up sprinklers used on flower beds normally have a pop-up height of 31 cm (12"), so can get over some small flowers / plants but often struggle with taller plants.
Micro-Sprinklers. For flower beds micro sprinklers are ideal, as they are cost effective to install and very flexible to move and add additional heads to when the planting develops. However, some clients don't like the look of them.
Drip irrigation. Drip irrigation has become more popular with the change in the water regulations. The best areas to use drip irrigation is to water hedge lines, large bushes, long thin beds and beds which you don't turn over very often.
Also drip line should be spaced at 30cm or 40cm across the bed. Unless the bed has good ground cover or is mulched, the drip line can be unattractive