Most people either follow up three times in a week or never follow up at all. Both come from the same place: treating the follow-up as a nag rather than a second favour.
The interval
Three to four weeks between touches is long enough that you are not applying pressure and short enough that they still remember you. For slow-cycle work — coaching, fractional roles — six weeks is better.
What goes in it
Never "just bumping this." Come back with something new: an article that is genuinely relevant to the thing they mentioned, an answer to a question they have since asked, a note that their competitor just did the thing they were considering.
When to stop
Three useful touches with no response means the timing is wrong, not that you need a fourth. Move them to a dormant status and come back in six months. The list is not scarce; your reputation is.
Start the habit this week.
Install it, run one search, and save five people. That is the whole first session.