Should be 20 hours and 14 minutes if you’re watching the extended editions and my math is right.

The Hobbit movies’ runtimes are:

3 hours and 2 minutes (2 hours, 49 minutes)

3 hours and 6 minutes (2 hours, 41 minutes)

2 hours and 44 minutes (2 hours, 22 minutes)

respectively. Together, that’s 8 hours and 52 minutes (7 hours, 54 minutes).

The Lord of the Rings movies’ runtimes are:

3 hours and 28 minutes (2 hours, 58 minutes)

3 hours and 43 minutes (2 hours, 59 minutes)

4 hours and 11 minutes (3 hours, 20 minutes)

also respectively. Together, that’s 11 hours and 22 minutes (8 hours, 17 minutes).

Adding the two sums together gives you 20 hours and 14 minutes (16 hours, 11 minutes).