A spore test (biological indicator) is the only check that proves your sterilizer actually killed the hardest thing to kill. Chemical indicators and the cycle printout show the conditions were reached; the spore test shows they worked.
The schedule
The CDC recommends biological monitoring at least weekly, and with every load that contains an implantable device. Many offices test more often, and some states require a specific interval, so your state's rule wins if it's stricter.
Run the test strip or vial inside a normal, full load, in the spot that's hardest for steam to reach, usually the center. Run a control that was not sterilized alongside it so you know the spores were viable to begin with.
When it fails
A positive result means spores survived. Don't panic and don't ignore it. Take the sterilizer out of service, review the cycle (loading, packaging, time, temperature, pressure), and retest. If the repeat test passes and you found an operator error, you can return the unit to service. If it fails again, the sterilizer stays down until it's serviced and passes three consecutive empty-chamber tests.
Items processed since the last negative test should be considered non-sterile. Recall and reprocess what you can, and document everything: the failure, the actions, and the retests. The log is what proves you handled it correctly.