How long does a scuba diving certification last?

There are no exact numbers, but PADI, the largest diver training organisation publishes statistics on its certifications, and it certifies approximately 950,000 divers per year. However, only about 550,000 of those are new divers - the rest are divers who are already certified but who have received further advanced training.

It has been estimated (by the NY Times) that PADI certifies about 70% of divers trained in the US, and about 55% of the divers trained worldwide. If we extrapolate those numbers, then that suggests that every year about 1,725,000 diving certifications are issued, of which approximately 1,000,000are new divers.

But obviously the more one extrapolates statistics and estimates, the fuzzier the maths gets, so I would treat those as very rough estimates.