The temporary certificates that InfoPath lets you make aren't really meant to be used in production. The ideal approach would be to get a certificate from a certificate authority, but I realize that can be a tricky proposition for some organizations.
In lieu of that, the Digital Certificate for VBA Projects utility that comes with Office (it should be in your Start menu) seems to create certificates with longer expiration times. I just tried creating one and it had an expiration date of 1/1/2018. You just create the certificate, and then select it in InfoPath.
How about giving that a try?