Plan a film night by calculating total runtime plus snack, stretch or debate breaks.
Start with 3 movies, 120 minutes each and 15-minute breaks. Then increase the break to 30 minutes. The movie runtimes did not change, but the finish time gets pushed by the repeated gaps.
When you have a total, open the Sleep Cycle Calculator. Pick tomorrow's wake-up time and see whether the marathon leaves enough room for a reasonable bedtime.
Use the related tools and guides when the first answer raises the next question.
The total is not only film length. Breaks between films can add a large hidden block of time.
Three 120-minute films with 15-minute breaks between them take 6 hours 30 minutes. Increase breaks and the finish time moves quickly.
Do not forget setup, food, pauses and credits if you are planning an actual watch party.