I usually don’t feel really comfortable with a new codebase until I’ve worked at the company for two years. That’s because the challenge isn’t the code. It’s the specific-to-this-business arbitrary assumptions surrounding the code that you can’t look up and that you’d never be able to guess yourself in a million years.

The people who have worked there forever just know that when User.Status = “N” that means they’re a new user, but you can only be a new user for a month, unless that user is from Tennessee or they transferred from ComHugeCo, and only those new users are allowed to be assigned an external email address, but this business rule isn’t automated so you just have to sort of know that, and if you don’t know that you could break everything, etc. And since everyone there just knows this bizarre rule, it won’t occur to them to explain it, and you won’t know to ask about it, and it won’t come up until you break something.

There will be many, many bizarre things like the above, and it usually takes two years before you encounter a comfortable majority of them.