Hello Carlo
The lifetime is the amount of time that a device will consider the specific router as its default router, unless informed otherwise. By default, this value is set to 1800 seconds, or 30 minutes. The RA advertisement interval, which is how often an RA is sent, is set to 30 seconds by default. Both the lifetime and the advertisement intervals can be modified on a per interface basis using the following commands respectively:
R1(config-if)# ipv6 nd ra interval ?
RA Interval (sec)
msec Interval in milliseconds
and
R1(config-if)# ipv6 nd ra lifetime ?
RA Lifetime (seconds)
As you can see, the interval can be configured with an accuracy of milliseconds, while the lifetime with an accuracy of seconds.
A lifetime value of 0, if configured, indicates to the client that this specific router is not available as a default router.
It’s interesting to note, that if you shutdown an interface of a router that is functioning as a default router, before it actually shuts down, it will send out an RA with a lifetime of 0, indicating gracefully to all concerned hosts that it is no longer available as a default router.
I hope this has been helpful!
Laz