Welcome to the forum.
If you're talking about money, then "on" is correct.
How much interest will I accrue on my savings?
I will have to pay 25% interest on my car loan.
If you're talking about someone liking something and wanting to know more about it, then it's always "in".
I'm interested in art.
Are you interested in that boy?
Is he interested in helping me on Sunday?
If you have "always" seen "interest on" in the second context I have quoted, then unfortunately you have always seen incorrect usage.