The value to be placed in the Preferred Lifetime field
in the Prefix Information option of Router Advertisements.
It is the length of time in seconds (relative to the time
the Router Advertisement is sent) that this prefix will
remain preferred, i.e. time until deprecation. A value of
0xffffffff represents infinity.
The address generated from a deprecated prefix should no
longer be used as a source address in new communications,
but packets received on such an interface are processed
as expected.