LONG and FOR A LONG TIME
LONG
Long is most common in questions and negative sentences, and after too and so.
Examples:
- How long did you wait?
- I didn't play for long.
- The concert was too long.
FOR A LONG TIME
In affirmative sentences, we usually use a long time.
Examples:
- I waited (for) a long time.(/ waited long is possible, but not usual)
- It takes a long time to get to her house.