The battery in my 4/4 seems to last in the order of 9-11 years, I have just put in my 4th battery since 1980. However my M3W battery did not make 3 years both are on Ctek chargers when not in use.

2021 M3W
2013 M3W - Sold
1971 4/4 2 Seater
1934 Sports