Financial Plan: Monthly changing MRR

Not sure if this is the correct subforum of StackOverflow, but let’s give it a try.

I’m currently creating a financial model for a startup, and the monthly recuring revenue per customer changes depending on the number months that specific customer has been a customer. Meaning that in the initial month, the customer purchases on average 0.5x products. On the second month, he purchases on average 1.5 new products, from thereon, he on purchases 3x products per a month for eternity (taking account 20% monthly churn, this eternity is in reality 5 months). How should one go about modeling this?

Thank you.