Totally depends on brand. I have seen as large as 1/16" under nominal on bulkhead type fittings. Usually you're 2/16's (aka 1/8") under nominal size - ex. -6AN will have a .25" bore.
In certain applications that are low psig but high flow I will ream an undersized fitting to be 1/16" under. Using best judgment of course as this will reduce your effective sealing surface.
Now, when it comes to aluminum hose ends you will almost always have an ID 2/16" (aka 1/8") under nominal. This is because a -6 (3/8") tube will have minimum 1/16" wall thickness.