I imagine its probably all over the place, to the point an average is not very useful, because on the one hand, something like a small cult that doesnt survive the death of its founders might last just a few decades, but something like Hinduism might last thousands of years and have a very unclear date to when it starts. You'd also have the question of when a religion ends exactly, like, one that has no followers left is probably dead, but what if it changes over time until the original form is unrecognizable? Is the original dead, or does the modern form count, and if the former, when did it end? Does it count as dead if a major world religion loses that status and becomes largely irrelevant, but still has a few small communities of followers, such as with zoroastrianism? If a religion does lose all of its followers, but people later attempt to recreate and convert to it from its surviving texts or similar, does it still count, or does the revival count as a new religion?

Maybe Im missing some obvious example, but I cant really think of cases, beyond the tiny cultlike ones, where a religion dies out organically either, most examples I can think of are cases where a religion is deliberately killed off, usually by another one supplanting it and having some conquering power or converted authority forcing its members to convert to the new one.

Both the Egyptian pantheon and the Hellenic pantheon went through multiple iterations across the ages of their respective peoples. Neptune was the all father originally, then Zeus was made the patriarch of the Olympians (while Kronos created Phanes who begat the cosmos). Curiously Aphrodite was Astarte before, and Ishtar before that, and didn't just bring love and beauty, but also the Phoenician alphabet which would replace Linear-B.

And the way Christian and Jewish scripture is interpreted today is very different than how it was interpreted in the 16th century, or the 11th century, or the 6th century.