Frequency Analysis

This is more maths/google juice that I'm after than a total solution, but my maths is rubbish.

I'm trying to do some churn analysis on a group of customers who make ad-hoc purchases, since they're ad-hoc it's difficult to say whether a particular customer has actually churned/gone to a competitor or just hasn't got round to making a purchase yet.

Purchase intervals will vary from annually to 100s per month.

What I'm trying to wrap my puny brain ultimately is creating a model to predict churn based on complaints, past purchase history, purchase value trends etc, but I've fallen at the first hurdle.

I simply want to work out how to rank a set of data based on descending frequency, weighted by recent date; so take purchases for example:
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Company C should rank lowest as there are purchases every month, Company K should rank highest as the drop is the largest.

Note: I'm not actually interested in ranking the actual values in excel per se, more learning how to identify in a much larger dataset - I'm going to have to ultimately convert the solution to SQL

I've been reading up on linear regression, which I think is the way to go, but I'm open to suggestions and new to this.