The validity time will indicate the time the granted quota is considered valid. There is no default
value for the Validity-Time. If the AVP is missing then the associated quota is valid indefinitely.
This is to save resources.
Imagine situation you are browsing i.e. facebook before falling asleep. You were granted let say 10mb.
Now the OCS has to remember it has session opened with you, it granted you quota and waits for consumption report.
Also PGW/GGSN who is counting this quota is assigning the resources to keep all this information in memory.
It's easier to assign the time during the quota is valid and then report it back to OCS and "forget" all the information about your session.