From related GGUS ticket https://ggus.eu/tech/ticket_show.php?ticket=88076:

This request is aimed at the notification sent out by voms admin when a
VO user has to sign the AUP again. Currently, there is only one email
sent to the user. You can specify in the configuration how much time the
user has to sign the AUP after the email was sent, but you can't have
multiple reminders and that is what Chris is asking for.
If only one email is sent to the user, there is an increased risk that
the user gets suspended because the email gets lost (spam filter, mail
server problems), or he can't act on it (holidays). Even though the user
can reinstate his membership any time by signing the AUP, an unexpected
suspension notice can still cause users to worry or even panic.
It would be good if voms admin could sent out multiple reminders. Chris
mentioned an example in his submission.

I think the best way of solving it would be to turn the
voms.aup.sign_aup_task_lifetime option into a comma separated list
containing the days at which emails are sent to the user before they get
suspended, for example

voms.aup.sign_aup_task_lifetime = 30,14,7,1

This example would send 4 emails, the first 30 days before suspension,
then one 14 days, one 7 days before, and the last one the day before
suspension. Configuring it in this way doesn't break compatibility with
existing configurations, e.g.
voms.aup.sign_aup_task_lifetime = 15
will still be a valid configuration, it will just send out one email 15
days before.