It really depends on what the reasons for this are. Were you asked to provide any additional information at the time of the interview? If not, you can always follow up with the local USCIS office as to the status of your application.
I would not call it normal, but it does happen for a number of reasons such as a backlog, incomplete background checks, etc. I suggest that you make an info pass appointment with your local immigration office and request an update on your file.
Not unusual, especially if they plan to send you a notice of intent to deny. That can delay a decision by 12-18 months, sometimes. If it goes for too long, you can file a lawsuit to compel action.
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