currently (as of version 2021.2), the validity period of auto-generated certificates cannot be changed in QuickOPC.
You can generate a certificate with longer validity period manually. See opclabs.doc-that.com/files/onlinedocs/QuickOpc/Latest/User%2...%20Instance%20Certificate.html
. If you use the same Subject Name, and place the certificate files it into the proper directories, it will be found by QuickOPC application and used.
QuickOPC applications issue a new auto-generated certificate when no instance certificate found. You can create a recurring job on the computer (e.g. using Task Scheduler) to simply delete the certificate files with desired period, and QuickOPC application will renew its certificate - as you suggested. Note, however, that this does not resolve the problem on the server you are connecting to - because it will most likely not trust the renewed certificate.
Certificate validity should be set based on security evaluation. One of the concerns is that the computing power of your model attackers should not be able to crack the certificate key during the certificate validity period. Given the current encryption algorithms and key sizes used, AFAIK 100 years is too long, as it would be possible to crack it sooner, using commercially available tools.