Will this hang from my factory rearview mirror?

Yes. Standard hanging design works with any stock rearview mirror bracket — just loop the attachment over and you're done. No tools needed.

How long does the scent last?

That depends on cabin temperature and how often you're driving. New car scent will gradually fade over weeks to months — flip it over or replace it when the smell dies off.

Can I use this on a windshield dash mount instead?

The product is designed for rearview hanging. A dash mount would require additional hardware not included — test-fit before committing to any non-standard placement.