I don’t know how long does the battery last but i got my financial calculator when i was in my undergrad and i am still using it now (original battery), it’s been 5 years+, just to give you an idea. Though i don’t use it everyday of course, but i did use it exclusively during my CFA studies.
I think if a calculator is working all the way up to the exam, there is very little chance for it to break, but buying a spare battery may be smart (eventhough i didn’t), bring an extra calculator if you already have one.
Best of luck!
NANA