Girls, How often should I get a bra fitting?

I have heard it said that we need to be fitted in a bra much more often than we are. I have worn the same bras (size) for three years now. Should I be fitted just to be fitted? Should I just measure myself or just go in? I don't feel all that uncomfortable I think, but I have heard that when you are wearing the right size you can tell the difference.

Girls, How often should I get a bra fitting?
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