Apple doesn’t break down its financial reporting by customer type, so it’s very hard to measure this over time. One of the very few data points Apple has provided on this was a comment on the September 2015 quarter earnings call last year:

We estimate that enterprise markets accounted for about $25 billion in annual Apple revenue in the last 12 months, up 40% over the prior year and they represent a major growth vector for the future.

That year’s total revenues were $233 billion, so enterprise was a little over 10% of the total. I don’t recall any updates since, but it gives you some sense of where that part of the business is — small fraction of the total business, but growing considerably faster and a major focus for Apple.

In this earlier piece, I talked about why this would be a bigger focus going forward:

At this point, Apple needs to make a strategic shift as it enters a new phase in the growth of these two products. The established strategies will continue to drive some growth, but it needs to augment these with new, more tactical, approaches to penetrate the remaining parts of the market where these established strategies won’t serve it as well. One obvious example of this is the enterprise, where Apple has so far fared very well simply through its existing strategies of creating compelling products for consumers and slowly adding support for enterprise use cases. But Apple will only grow so far in the enterprise as long as IT departments support Apple devices reluctantly rather than wholeheartedly, and the IBM deal is a way to change that. It won’t move iPhone sales by a huge percentage, but it’s one of many smaller strategic moves that Apple will have to execute on to drive the next phase of iPhone growth.