According to Simon Blackburn, it’s a lot.

The sad thing is, David Hume is an easy read. I don’t mean that his ideas are necessarily easy to fully comprehend nor that you can breeze through him, but that his writing style is clear and concise, which is all the more amazing because he was writing more than two hundred and fifty years ago (we are almost at the end of the third centenary of his birth). His An Enquiry Concerning Human Understanding (generally considered the foundational book for his particular brand of philosophical empiricism) is a wonderfully lucid read compared to your average philosophical tome (more so even than Locke) and it’s sad that it’s not more widely read as a means to provide a solid grounding in a significant thinker and provide one with, for lack of a better term, ‘food for thought.’