How deep do I need to make the post footings for a residential deck in Northern Michigan?
The post depth depends on the frost line. The size of the footing depends on the soil type under the footing.
You need to dig until at least under the frost line, and then, if you are not on rock or undisturbed/compacted soil, dig until you get there. You can analyze the soil as you dig. The size of the pad of your footing depends on the soil: gravel, sand, or clay. See also https://www.decks.com/how-to/articles/soil-conditions-for-deck-footings
Check with your local municipality, they have this info for you.
Here's a map of the frost line (depths), and you'll see that Michigan is around 55&quot;.

From https://www.decks.com/how-to/articles/deck-footing-frost-depth-map :

Always check with your local building inspections department to see what is required in your area. Furthermore, you must install footings at least 12” below unsettled soil. If you are building into the side of a steep hill or beside a retaining wall you will have to maintain the minimum distance to grade not only vertically but also laterally. In order to achieve a suitable lateral barrier you may need to dig your foundation much deeper than on a flat surface.