I am hyper allergic to scented products and I cannot be around people who use them.

When interviewing for a job, at what point to do I mention this? How do I bring this up?

Are you interviewing for a position where it would be avoidable? In my opinion bringing it up in an interview would be a negative.

I am not allergic to it but I do hate it. The woman that empties our trash daily fills the bathtub with perfume and swan dives into it. The stink lingers for a long time.

I would not since who hires someone who cannot interact with the people they may or may not have an allergic reaction to. The onus would be on you to find ways to deal with your allergies effectively. It would just be to disruptive. Is this treatable? What kind of position are you interviewing for? How have you dealt with this issue before?

You have to admit that your condition limits your opportunities, unless you take some kind of action to deal with it.