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There are escalating time limits on incorrect PIN attempts, which is also enforced in hardware by the Secure Enclave in A7 chips and above. This would mean even breaking a 4-digit pin code without that delay being removed would take a long time. Additionally the device may be set to wipe after 10 incorrect attempts.

Attempts | Delay

1-4: none

5: 1 minute

6: 5 minutes

7-8: 15 minutes

9: 1 hour

Source: https://www.apple.com/business/docs/iOS_Security_Guide.pdf

Secure Enclave is _not_ hardware; despite being isolated from the main OS and CPU, it is still software-based and accepts software updates signed by Apple.

I believe the default on new installs is now 6, but options for a more complex PIN, either numeric or straight up alphanumeric, are available.

I use an alphanumeric password on all my iOS devices