How many seconds is the OL expected to give the QB on ping downs?

If I recall correctly, the OL is expected to give its QB 3-5 seconds of protection so the QB can attempt to p the ball. Am I correct on that umption? If so, is it the QB's fault if he is sacked an average of 3 times per game in under 3 seconds or is it the Offensive Line's fault? I can see it being part of the QB's fault if a blitz was coming as the QB is expect to adjust the blocking scheme, but lets say this is based on no blitzes and a 4 man front.