Jambo, It depends on the leak rate. One of FRs got to just shy of FL310 before the cabin horn went off.
Interestingly, a Boeing Tech Rep came to visit our company for an annual road show. It was stated that since the Helios Crash that there were planned changes to the pressurisation system in the pipeline with "Voices and Flashing lights" instead of, and perhaps in conjuction with the Cabin/Config. horn.
As for airmanship this has happened to some very crusty old veterans on the 737 with time in type well in to the 10Ks. I certainly would never be so bold as to say it would never happen to me, although as a company we were extensively educated to the dangers and flaws of the system after our incidents. We have robust procedures now which if followed give about 8 opportunities to trap the error before one has painted oneself into a corner.