Hello Dean
Wow how long is a piece of string ?
or how many kilometers per litre ?
Good reading here
http://bdbatteries.com/marinegraphs.php
I live rural and the folks just put batteries in with mixed capacities and chemistry with unpredictable results
Rule of thumb Deep Cycle AGM batteries lifespan are generally rated to 50% discharge for 800 charges
i.e 12V 100Ah AGM discharged 50% to 50Ah = 800 charges
i.e 12V 100Ah AGM discharged 25% to 75Ah = 1200 charges
i.e 12V 100Ah AGM discharged 75% to 25Ah = 400 charges
Hope that helps