What is the grace period for Allstate home owners insurance?

There is not a grace period for an expired home insurance Policy. They expire on the anniversary date unless you choose to renew the policy
There is not a grace period for placing a home insurance claim in the United States.

All home insurance policies require in various policy language that a home owner report a covered loss as soon as practically possible and that the home owner take any necessary mitigating action to limit further damages from a covered loss.

Each U.S. state also has it's own regulations that indicate how long after a loss an insured may report a claim or a covered loss to the insurance company. In the United states this is generally 1 to 2 years from the date of loss. It just depends on your states regulations.

The best source for information will be your insurer or your insurance agent. They can usually review your coverage and it's limitations with you over the phone.