limit_req_zone key cache lifetime

[Chad Hansen]

I'm looking for documentation or explanation for how keys expire in the
limit_req_zone. I have the basic documenations here:

*A client IP address serves as a key. Note that instead of $remote_addr,
the $binary_remote_addr variable is used here.
The $binary_remote_addr variable’s size is always 4 bytes for
IPv4 addresses or 16 bytes for IPv6 addresses. The stored state always
occupies 64 bytes on 32-bit platforms and 128 bytes on 64-bit platforms.
One megabyte zone can keep about 16 thousand 64-byte states or about 8
thousand 128-byte states. If the zone storage is exhausted, the server will
return the 503 (Service Temporarily Unavailable) error to all further
requests.*

But there's no explanation for how the key cache eventually clears itself.
Is any available?
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