Customers and the gap between sales

Im trying to model a difficult question and cant grasp a solution for this so hopefully the community can assist. 

In my customer sales history I want to track the length of time between the last sale and the next to last sale to determine if this customer is what I would call "New". So if the gap between my current sale and the next previous sale is two years on a rolling basis I would flag the customer as "New". 

Also if I have never sold to this customer before I would flag them as "New". 

We have customers that do business with us, then disappear then come back, but if the gap in time is more than 24 month on a rolling basis we consider them new. Any ideas on how to solve this... help is greatly appreciated.