I dont want my calc battery to dry in middle of the exam… any idea how long can the pre installed battery run???
I have been using them for the past 2 years unchanged and no issues.
Why take a risk. A week before the exam change your battery and make sure it works. Have a spare battery with you anyway!
4.5 years and still running… 
10 years. Not kidding. I have two though that I rotate with.
replacing the battery takes 2 seconds… just bring a spare to the exam
5 years on mine. I bring a backup though.
Long enough to fail each level at least 3 times without having to worry about your calculators…but if it’s keeping you awake at night, simply bring an extra battery to the test center…and problem solved