It's entirely group dependent (Midnight Mirror is very investigation-based, so it'll depend on how long it takes your group to put two and two together). When I ran it, it took ~35 hours, but my group plays online and we're pretty loose when it comes to "hey I need to grab food, give me a minute" types of breaks, so you could easily shave several hours off of that time.
|Brew City Crafter|
To add a data point, I ran From Shore to Sea last month for a party of five. Played nearly 11 hours, and I ended up needing to skip most of the encounters in the final area so we could finish before the location closed. Also came near a TPK at one point due to a nasty encounter with 8 legs.
When I ran From Shore to Sea a few years back, whenever I was meant to run a random encounter, I made the party cavalier, who consistently dealt more damage than any enemy had HP, roll initiative against the enemy and see who won. Every time the cavalier won, I ruled that he squished the enemy in one hit and the encounter was skipped.
Pretty late reply, but anyway: I played Mignight Mirror in two PFs slots, so about 10 hours. First session was pretty relaxed and a slow pace, second session I had to push them slightly. If I had kept the same pace as the first, it would probably have been 11-12 hours. So, depending on how fast you run through it, 9-12 hours seems right.
We ran from Shore to Sea at a con last year, When people sat down they were warned that we were under a time crunch, so we will try ot keep you on topic, and to try not to waste time during your turn. It was scheduled over 2x 4 hour slots + the 30 minute 'travel/lunch' time in between. It ended up going 20 minutes over time, and they skipped two encounters.
Midnight Mirror took my group about 30 hours to complete. There are a few red herrings in there, which I happily let them explore.
If you point out more clearly how to proceed, I am sure it can be done in about 10 to 15 hours. If you want less than that you would have to cut a significant amount of content.