In German Language and Usage we just had the question German analog for “That's no rocket science”. As a native German Speaker, I do not know which situations this phrase is used in. I understand it means the problem isn't too difficult. But can I conclude something about the time needed to solve the problem? Can it for example be used when simple and almost boring repeated effort is needed to solve the problem?
Your understanding is correct. "That's no rocket science" simply means "That problem is easy or straightforward." In other words, you do not have to be a rocket scientist to solve it. It does not really carry any more information than that.
Incidentally "He is no rocket scientist" usually means "He is not exactly dumb, but not too bright either".
In addition to the usages given by Dima there are a few creative implications to using this phrase.
For example you can use it to suggest that normal Joe could do something himself. "Come on Joe just get in there and do it, it's not rocket science."
It is also used in reference to past scenarios that mystify people but all the clues are there and they just aren't thinking straight. You can imply that they should have thought of that themselves by saying "it didn't take a rocket scientist to figure it out ..."
The phrase "That's no rocket science" or "It's not rocket science" refers to activities that are not complicated, and thus aren't difficult to do.
It does not necessarily refer to the time one takes to complete the task.
Cleaning a car properly may take two hours, but it's not rocket science (you just follow the steps one by one, and it's easily doable, even though it may take some time.)
Deleting an email is also not rocket science (you click once, and it's done).
In other words, the phrase generally refers to any activity that does not take too much "brain" work to complete, no matter how long it takes to do.
It's used often to put down or make light of a task/situation:
A: "Can you help me do this-task-that-is-difficult-for-me?
B: "Oh, just go do it. It's not rocket science. Go figure it out!"