What is the estimated life span of natural building materials?

Straw bale as with mud construction will last indefinitely provided one has followed good building practice. Some of the earliest constructed straw bale buildings i.e. the earliest known surviving straw bale house - the Burke house was built in 1903 in Nebraska and still survives in good condition despite having been abandoned for many years. Remember straw bale only began with the advent of the baling machine!

There are numerous examples of very old mud constructions having survived to this day besides our relatively young 300-400 year old Cape Manor houses from the 500 year old cob buildings in England and medieval tudor frame and clay straw buildings of Europe, there are 7 - 9 storey skyscapers in Yemen many over 1000 years old and parts of the great wall of China which are many thousands of years old!