How long does an 80g brick last?

Solid bricks like this typically last weeks to a couple months depending on heat and airflow in your cabin. Scent will gradually fade as the brick is consumed.

Does this come with a mirror hanger?

The brick itself comes ready to hang or place. If you need a mirror hook attachment, you'll want to source one separately — any standard car air freshener hanger will work.

Will the scent overwhelm a small car interior?

New car scent air fresheners are designed for standard cabin volumes. Start with the brick in place for a day or two — if it's too strong, move it to the back seat or remove it temporarily until the scent settles.