First I think you have to define, 'learned Japanese'.
I once met a person who said I spoke Japanese well. I said," Thanks, but my reading is terrible. Can you read Japanese?" And he said Yes. A straight yes. He wasn't lying. He could read Japanese, but he could only read Hiragana.
Did he learn Japanese? Yes. Has he learned Japanese? I think, no. He has just "learned some Japanese" or "knows some(very little) Japanese".
To say that you "learned Japanese" I believe you need at the very least an Intermediate level (chart below) in both speaking and reading.
But you don't really know Japanese until you've reached Upper Intermediate.
The Levels of Speaking Japanese
Beginner
Being able to pick up a girl(guy) in roppongi who doesn't speak a word of English. This only requires you to know only a few phases so it is the first step.
Intermediate
Have Japanese friends who can speak some English, but will still speak to you in Japanese. This varies much on the Japanese person's English skill. If it's very high and he or she still wishes to speak to you in Japanese rather than English this could be closer to an Upper Intermediate level. This could also depend on how many other people in the group understand Japanese or English.
Upper Intermediate
Able to listen to the news with 80% or more comprehension and talk about with in Japanese. I say 80% because even native speakers don't understand 100%.
Expert
Able to bullshit your way through a topic that you have no idea about, of course in Japanese and without getting caught. It's easy to talk about something that you know. However, even in your native tongue it's difficult to something you know nothing about.
The Levels of Reading Japanese
Beginner
Read Hiragana and Katakana
This is really no big deal. Just reading them individually can be learned in 1 week if someone put their mind to it. If you can read them with good rhythm you are closer to Intermediate.
Intermediate
Read 6th grade Kanji
Upper Intermediate
Read a Newspaper with 80%+ comprehension.
Expert
Read Non-fiction and literature.
I just thought up this chart now so it's probably full of errors
! There are other factors, as well. How much of the culture and history that you know and maybe your accent.