If the surface is already painted, you should wait until the surface appears completely dry. On a warm, sunny day, this usually takes between 2 to 4 hours.
If you are planning to paint raw wood, you should wait a full day for the surface to dry. Water may soak within bare wood. If you paint too soon, you may trap the water within. This can lead to paint failure.
Pressure washers often eliminate chipping and peeling paint, leading to exposure of bare wood. Because of this, your best bet is to wait a full day before painting, unless you are working on a metallic, plastic, fiberglass or vinyl surface. If you are painting a non-wood surface, you can begin work as soon as it appears dry.