Depends on what you would consider adequate coverage, and if you are considering both FSX and FS2004.
Assuming that it is both sims, and adequate coverage for a region entails accurate mesh, landclass, and at least 75% of the region's/county's airports are customized, I would say...
around 85%....maybe 90% of the world is covered by sufficient addon scenery today. The bulk of that is made up of FS2004 addon sceneries however.
