How much of the world's forest have we destroyed?
About 50% of the original forests, pre human activity.
Forests have been slashed and burned by humans for probably thousands of years, but its only in the last few hundred years that this is becoming more problematic (see pic 1).
This has proceeded over time. First, much of Europe was deforested hundreds or up to a thousand years ago (some of these forests are coming back), next much of the U.S. and parts of Canada were deforested during the 19th and 20th centuries. Some of the U.S. Eastern region forests are coming back. Deforestation hotspots today are predominate in tropical countries (Amazon, Indonesia/Malaysia, China, Congo/Africa). See pic 2.