Some marketing campaigns include an arc where each contact gradually moves the customer along a path.
Welcome Campaigns use this heavily, along with email courses ("opt-in to learn 9 ways to make your widgets go bang").
In those it's usually clear when you should start selling harder. Usually near the end once the customer has been made aware of the problem and your product is the solution.
Other campaigns don't have a clear path and you could drop in a sales message or two anywhere. But where?
If the campaign is going to new potential customers, anywhere you want. Sooner is probably better (a customer who never buys isn't a customer).
For existing customers though, you'll want to be more careful. Sending sales messages too early could make your store appear greedy and only be interested in the transaction.
(Same for if your lists include a mix of potential and existing customers. In that case, err on the safe side and pretend it's a list of existing customers)
Repeat customers exhibit repurchasing behavior, meaning you can track when they repurchase. Using the delay between in-between purchases will tell you how quickly they are naturally ready to buy again.
This value, called the Customer Purchase Latency or just Latency, is the ideal starting point for any sales messages.
If you measure your customers and they are purchasing every 90 days on average, a sales message around that point of time would be well-received.
I recommend starting a little earlier, say 10-20% earlier, since this is the average and you want to nudge behavior to be above-average. Using the example above, you'd schedule a sales message 72-81 days after their last purchase. This can shift their purchasing behavior to happen sooner, which means more orders for your business.
Calculating your Customer Purchase Latency isn't difficult (it's just addition and division) but it can take a lot of number crunching if you have even a small number of customers. Repeat Customer Insights will calculate it for you including how it changes based on the sales channel.
However you do it, timing your campaigns to this customer behavior is ideal and will help your Shopify store improve with repeat customers.
See how the month a customer orders will change their behavior
Repeat Customer Insights will automatically group your customers into cohorts based on when they first purchased. This will let you see how the date customers bought would impact their behavior.