The plants we sell have lived in their pots for many months and often have been potted on for years, we keep them well watered and drained, it's not in our interest to send out poor plants.
We recommend you water your newly planted at least every other day in dry weather for it's first spring/summer seasons.
However, just as you can under-water, you can over-water it too if they are surrounded too close with clay soil. So it is important when planting to always dig out twice as wide and twice as deep removing clay and bricks to giving the roots something to work with whilst getting established.
See image below of an over-watered Photinia Red Robin in clay soil, the customer saw the leaves drooping, so the more they watered it.
They have since dug it up finding 'the original rootball was surrounded by an underground water filled moat' so they 'removed lots of blue clay and claggy soil and bought new bags of topsoil to fill the hole'.
This Photinia had been caught in time and making a full recovery.