A cracked windshield on a 737 is quite rare. Most likely reason for a L1/R1 window change is fine scratches in the windshield which obstructs vision, caused by the windshield wipers. Another less common reason is electrical arcing within the windshield.
As I recall, the R&R procedure takes about 60-90 minutes and is a non-event for for the tech folks.
Not on the 737, more like 4-6 hours.