When it comes to watering trees, shrubs, and flowers there is not a simple formula. Temperature, rainfall, soil type, amount of sun/shade, and mulch will all play a role in the watering schedule of plants. A key factor, however, is the soil texture. Clay soils can hold up to three times more water than sandy soils. 

Check soil by moving stone or mulch aside and feeling the soil for moisture is the quickest and easiest way to know if you need to water again. You want moist soil, not wet. If there is little or no moisture in the soil you should water more frequently. When watering is needed, water to soak the soil deeply, not just to get it wet. Deep watering leads to a desirable long root system and less watering. One way is to set a hose on trickle and let it sit at the base of the plant for 15 to 20 minutes for trees. With landscape plants its best to individually water the plant and the surrounding soil. When I water plants the hose is moving plant to plant every 10-15 seconds to allow the water to soak in for about a minute, then watering the same plant three times. Start at one end and work your way around. On hot summer days you might have to water in the morning and evening. In the spring or fall it might be once a day or every other day. It typically takes 3 weeks for plants to start rooting in the soil and be able to go without watering for a week. These are general guidelines as each project is unique.