If you consistently have clean diagnostic reports you do not need to run them as often.
In general, if you run clean diagnostics for a full month, with no errors, you can increase your Diagnostic Period Reminder to 14 days. If they continue to remain clean, increase the period to 30 days.
You can change the Diagnostic Period Reminder by going to Maintenance > Preferences. Edit Preference #1.
It is not recommended that you increase the period reminder to over 30 days. You will them lose the advantage that the PERFECT FIT Diagnostic Report provides by checking for data integrity.