You can actually look this up here on the AAPC web site, under 'Resources', choose 'E/M Analyzer'. Alternatively, it's published by Medicare each year. The numbers are Medicare data only, so of course could be different for practices that see a high percentage of non-Medicare patients.
For Internal Medicine, the percentages are:
level 3: 40.5%, level 4: 49.2%, and level 5: 4.5% (established) or
level 3: 29.7%, level 4: 48.1%, and level 5: 16.5% (new)