At some point it's not worth it to market to some customers for reorders.
They might be unresponsive.
They might be chronic returners.
Or they might just cost too much to reach so even when they do order, you're not turning a profit.
Your limit will depend on how much it costs to reach them. If you focus on inexpensive contact methods like emails without discounting, you'll want to stay in contact for longer than if you gave deeper discounts or ship out physical catalogs.
You can use RFM modeling to get a rough idea too. It doesn't look at costs but pits all customers against each other to figure out who are the better customers. With it's scoring the lower numbers are most likely the groups to avoid. They'd be the customers who have gone the longest without an order, have only a single order, or have spent the least over their lifetime.
Of course a customer who hits all those of those is going to be your worst.
Repeat Customer Insights will use an RFM algorithm to automatically score and grade your customers. The Customer Grids and Automatic Segments provide a more visual way to find these customers.
Customer behavior analysis for better Shopify store performance
The Shopify App that increases repeat customer purchases through customer behavior analysis.