With a bad alternator, a car can run until the battery dies. Driving with a bad alternator can leave you stranded!
The alternator charges your battery while you drive. Without a working alternator, your battery will quickly drain while in operation. You’ll likely get 25 miles on a fully charged battery without a working alternator. Once the battery runs out, your car will die (or won’t start back up).
You should head to a mechanic as soon as possible if you’re having issues with your alternator.