The frequency of the reminder is controlled by the pending users reminder delay
site setting. The default value of that setting is 8 hours. Try setting it to a lower value. The pending user notifications are triggered by a background job that is run once per hour. This means that setting the pending users reminder delay
to 0 will still cause the notifications to only be generated once per hour.
Note that these notifications are only sent to site moderators, not to site admins.