Here are some relevant posts, I’m sure there are more, and maybe more relevant posts as well…
https://meta.discourse.org/t/notification-options/21694/2?u=watchmanmonitor
My thoughts on the matter is that getting the UI to allow this is no small feat, even if it was on the priority list.
The API & Webhooks are enough to allow the automation of messages like this. I don’t think it would be too hard to setup a webhook tell Zapier (or your app, etc) there’s a new person, and have Zapier automatically create a PM from you to the new user after X days.
You’ll be able to tailor it to your liking far sooner than waiting for someone to build in a UI for arbitrary timed messages.