Generally takes 2 to 3 days, depending on atmospheric conditions for the deterioration of the paintwork to become noticeable.
In warmer drier conditions, such as now, the process takes a little longer. You'll first notice it on those cars where oxidation has previously occured. Any further ingress allows the water to further cause paintwork deterioration.
Once the water has been in contact with the paintwork surface usually only a thorough application of a paintwork restorer will be sufficient to stop any further damage, this is usually achieved by using a paint restoration product such as T-Cut.
Good luck.