How to measure churn in non-SaaS environment?

What is the best method for measuring churn in a marketplace model? Our customers can sign up to access our platform without paying a subscription fee. Our revenue then comes from a particular end user execution. A SaaS model may come as the company matures but need to work with what's in place at the time being. Would you simply tie it to non-usage? e.g. 60 days of no logins would be flagged as a churned customer?