Whats the minimum amount needed when leaving breadcrumbs for potential future mates.

Currently leaving trails for 3 girls, I'm getting different engagement from each and wanted to know everyone's thoughts.

First girl: We hit it off over the summer then she took off to another city for school. We talk about every 1 month briefly, so far this has likely landed me a meetup over the Christmas break before shes back to school again. What can I during this brief meeting to keep this flame lit until next spring comes along? Is it worth making a move or does that make it look like your just looking for a quick fling?

Second girl: I see her briefly at the hardware store and chat her up everytime I'm in for a new tool for my Buisness (about once a week). She seems engaged and flirty so this breadcrumbing seems like it's going to be able to land me a date in the future if I want.

Third girl: meet once and really hit it off out of town, I flew a small plane around the bay with her as my co-pilot and then we watched a movie. Unlikely anything happens here as she lives out of town but she seems to enjoy me leaving breadcrumbs behind over text a few times a week.

What is everyone's thoughts on leaving breadcrumbs in general? When is this strategy successful? When does it fail and how long can you continue this before the flame disappears usually? Are there any specific techniques when it comes to breadcrumbing?