Depends on:
Size and type of pack
How many people are working on it
Do they bother to test and fix it
Ages ago I made modules & whole game for Bioware's "Neverwinter Nights". I quickly discovered that the testing, fixing, testing some more, and so on took more time and effort than the initial construction. All it takes is for one (1) character in all that code to be missing or wrong to wind up with a mess. And that was a comparatively simple engine. To the question, I'd say at least a year to have even a chance for things to be right.
