Currently (June 2016), the validity period is expanding even further to 8 hours based on VagrantCloud documentation :
Session sessions expire in a short time (currently 8 hours), but can also expire manually using ctrl-c from an exchange computer or through a web interface.
However, it does not seem to need to set the expiration interval yourself. And I do not believe that it will ever be. The goal of vagrant share is to share your work for a limited time only so that your client can watch or for your partners to do some testing and come back with feedback, not .
source share