B737 Pilot reported discovering loose bolts on the cockpit windows during maintenance at the gate and suspected the aircraft had been flown in this condition for multiple days. The aircraft was removed from service for repair.

Narrative

Upon arrival at the aircraft; the inbound crew informed us that there was a small maintenance discrepancy with the APU 'OFF BUSS' light. Maintenance had been called. Once in the cockpit the ground crew personnel; asked through hand signals to disconnect ground power. I then opened the R2 window to inform them directly via voice that we needed to wait until maintenance was finished with their item before doing so. At that point I notice all the bolts on the window Column between R1 and R2 windows were completely loose and not torqued down; to the point that one bolt fell out when touched. These bolts are normally covered when the windows is in the closed and locked position. When I pointed this out to the mechanic; he had stated how long he had worked here and that he had never seen anything like that before. The aircraft was taken out of service.To which it had flown multiple days with this error.Cause: I was able to trap this threat because I needed to open the window to communicate with ground personnel. Suggestions: Maintenance needs strict adherence to the proper procedures for changing a windscreen per the AMM (aircraft maintenance manual). The crew chief should have caught these loose bolts prior to returning the aircraft to service.