- How long have you been here?
- How long have you been in here?
There really isn't that significant of a difference.
Using the sentence with "in", indicates that you are in(side) a confined area. Usually used when talking about the place being displeasing (example: Jail).
- I have to go.
- I have to get going.
Again, not really that much of a difference.
"I have to go" would be used when you are letting someone know you are immediately leaving.
"I have to get going" would be used (a nicer way) when you are letting someone know you NEED to be immediately leaving.