We get an average revenue of about $100 per user, per month - given our high ARPU, what kind of average customer lifetime should we consider a "healthy" amount?
The answer depends on how much does it cost to acquire customers and other costs to run the business.
You want to have a profit on your customer's lifetime value (LTV, which is the ARPU * average contract duration) , taking into consideration the average cost to acquire customers and the cost to run everything else on your business.
This is a good reference: https://www.forentrepreneurs.com/saas-metrics/