When you install Directory Opus for the first time you are running on a Stock certificate. This certificate is not an evaluation certificate. The Stock certificate only lasts for 30 days and the licence manager may appear from time to time (especially towards the end of the period) to remind you.
To evaluate Opus for an additional 60-days, and also delay the reminders, you can request an Evaluation certificate. You can do this by clicking the button in the Licence Manager.