There's a lot of variables that determine battery life so the only reliable gauge is how long your previous battery lasted. This is assuming you haven't changed your driving habits or place of residence.

I just changed my battery last week on my 330xi. It lasted 10 years. I don't do many short trips or sit parked with the stereo on. Cranked up perfectly fine last winter down to -30 C. As Md said, trickle charge your new battery when you get it. My new Exide 94R was on a smart charger for about 8 hours before it registered "full".

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