Six months really isn't uncommon at all. Almost every company has a weird legacy codebase you have to figure out. Unfortunately it doesn't generalize very well -- figuring out why some weird COM interface has 14 inheritance dependencies or what weird JSON file is driving behavior is part of the job. It's just, uh, the part that kind of sucks.
Depends on developer I one took over a super nich system written in a new language (Pl1g) and using bleeding edge tech (Map reduce in the 80s) took me < than a month to get productive.
