If you are using Lightroom, then I suggest that it is not the folder structure alone you should be relying on to manage your image library. However it depends on the number of images you have in your catalogue and you may find a folder structure adequate for your needs.
With a folder based system, the degree to which you can successfully retrieve images depends on your ability to memorize the folder structure and to remember where everything is stored. This will soon become unwieldy for a large archive of images.
However, file management by metadata, in which you search for a file by its attributes rather than trying to remember in which folders you put the pictures, is the best way to make use of Lightroom's "digital management asset system". You can no doubt find plenty of information on DAM systems for Lightroom on the web. For me it boils down to remembering to use keywords on importing. Later if I wish to retrieve photos relating to any of those keywords, then using the simple search text function, instantly brings up all the associated images, no matter in which folder they happen to be.
To answer your direct question, as for my folder structure, I use a simple annual numbering system, "myname_year_sequentialnumber" and put them in a folder for that year. Therefore I have only a folder for each year. That may be too simple for many people, but provided I have applied relevant keywords, the folder system doesn't really matter. Thus the keyword "bird" will generate images of birds from the whole library, or images of only birds/type of bird, (e.g. keywords bird, robin) will bring up only robins.
If I have been abroad, then the keywords for countries, towns, etc, are all that is necessary to find the images.
Other metadata is also important, such as EXIF data, to discriminate by e.g. particular lenses or by GPS, which should be relevant for professional photographers.
Apologies if you are not using Lightroom, but I suggest the same principles apply to any digital asset management system. Hope that helps.
