Two to four follow-ups over 2–3 weeks captures most of the replies a sequence will ever get. A large share of total responses come from follow-ups, but returns collapse after the fourth touch and irritation compounds.
The craft that keeps them welcome: each follow-up adds something (a resource, a specific idea for THEIR situation, a relevant example) instead of 'just bumping this.' The bump email is the annoying one. It asks them to do your remembering.
End cleanly: a final 'closing the loop. Reply anytime if this becomes relevant' note leaves the door open and the impression intact. Then actually stop; the eighth follow-up has never once been the charm.