A good maintained bat , must come for 5 year , then it will last a year or more on continuous running , else a idle car will loose soon as after 5 year the cell life comes down.
One of my experience ,
1. One of my friend has a petrol uno , it has 37 amp bat as stock and accordingly it has a alternator. he change it to a 2nd hand indica bat (my indigo has 60 amp must be same for indica) , it lasted for 4 year to him. It was in indica for 4 years. So that amaron bat lasted for 8 year. On continuous running , it cranked the car well.
One of my query is ,doesn't that alternator ruin the battery as it has higher amp? It doesn't properly charge the bat.
2. In our omni it was for 4 years then we sold it . still its running in that bat