Captain Vortex,. .The correct procedure for replacing a B737 flight deck forward window will be in accordance with the relevant aircraft maintenance manual. . .I'm no expert on the B737, but in comparison to types I'm familiar with, I think its unlikely that you can fly with it broken or cracked. The only defect usually allowable on windscreens are delaminations and they are only allowable within MM limits.. .Regarding time to change a windscreen; well, a good engineer SHOULD take as long as is necessary to ensure that the replacement is carried out IAW the MM and be properly certified.(Remember the BA BAC1-11?) However, usually you have managers/despatchers/passengers/accountants looking over your shoulder providing "commercial" pressure and therefore the time factor is down to the professional integrity of the engineer.