A reminder every 3 to 6 months is excessive. Once a gateway is set up and working, there's just no reason to log in ... ever. So sending emails when there's nothing wrong is a sure way to train people to ignore the emails.
Suggestion: Do what commercial subscription companies do. Send an email only when action is needed and do it with increasing frequency. What worked well at my last company was: 60, 30, 15, 7, 1 days.
Michael N6MEF
At my request, we're cleaning out obsolete entries from the portal.
A reminder is sent after 3 and 6 months with no login.
A warning is sent after 9 months with no login.
A notice is sent after 12 months with no login.
After 15 months, the user, their network allocations, and any gateways belonging to them are deleted.