This reports the configured maximum lifetime, in seconds, this security association is allowed to remain active before it must be rekeyed or torn down. It's the ceiling that ntqVPNLifetime is measured against, and admins use the pair together to predict roughly when the next rekey negotiation for this tunnel will occur. A max lifetime of 3600 seconds means this SA rekeys hourly, which an admin would account for when correlating brief traffic blips with expected rekey events.