"Document IDs consist of two parts, a prefix (that is randomly generated per site collection by default) and two sets of numbers. An example Document ID looks like this: J2W3DN6QF6XW-2-10. The first number is the ID of the list that the document was first created in and the second number is the ID of the item in that list. The prefix can be specified per site collection and can easily be changed from the Document ID settings page."
Source: Document ID in SharePoint Server 2010
According to that description, only the minimal length of a default document ID is for sure, which is 16 (e.g. ABCDEFGHIJKL-1-1).
The actual length depends on the number of items in your library and the number of libraries on your site.
You can also modify the prefix yourself and choose a string of 4 to 12 characters which would decrease the possible minimum length to 8 (e.g. ABCD-1-1).