As of 2016, which big screen zombie apocalypse movie has the most screentime (in terms of minutes) in the movies? Please include sequels and spinoffs as long as it's the same apocalypse.
TL;DR: The series that began with Night of the Living Dead is probably the longest.
George A. Romero's installments:
Night of the Living Dead (1968): 96 minutes
Dawn of the Dead (1978): 127 minutes
Day of the Dead (1985): 100 minutes
Land of the Dead (2005): 93 minutes
Diary of the Dead (2007): 92 minutes
Survival of the Dead (2009): 90 minutes
Total: 598 minutes/9 hours, 58 minutes
If we add in the other movies that were made with input from George A. Romero or his co-writer on Night of the Living Dead, John A. Russo, the total is even more impressive:
George A. Romero-affiliated installments:
Night of the Living Dead (1990): 92 minutes1
Dawn of the Dead (2004): 100 minutes1
Day of the Dead (2008): 86 minutes1
Subtotal: 278 minutes/4 hours, 38 minutes
John A. Russo's installments:
Return of the Living Dead (1985): 91 minutes2
Return of the Living Dead 2 (1988): 89 minutes2
Return of the Living Dead 3 (1993): 97 minutes2
Children of the Living Dead (2001): 90 minutes2
Return of the Living Dead - Necropolis (2005): 89 minutes2
Return of the Living Dead - Rave to the Grave (2005): 94 minutes2
Subtotal: 550 minutes/9 hours, 10 minutes (not including the 96 minute long Night of the Living Dead)
Total: 828 minutes/13 hours, 48 minutes
Grand total: 1,426 minutes/23 hours, 46 minutes
And if we add in totally unauthorized remakes which appeared because the original Night of the Living Dead was never properly copyrighted, and is thus considered to be in the public domain, we expand the total screentime even more:
Night of the Living Dead 3D (2006): 80 minutes
Night of the Living Dead - Resurrection (2012): 86 minutes
A Night of the Living Dead (2014): 79 minutes
Night of the Living Dead - Genesis (2016): 135 minutes3
Night of the Living Dead - Rebirth (2017): 90 minutes4
Night of the Living Dead 2 - Deadlock (2017): 90 minutes4
Night of the Living Dead - Contagion (TBD): 89 minutes
Total: 649 minutes/ 10 hours, 49 minutes
Grand Total: 2,075 minutes/34 hours, 35 minutes
The question is limited to film, and excludes television series, but if this weren't the case, the winner would be The Walking Dead.
The Walking Dead (through season 7, episode 6): 4,440 minutes/3 days, 2 hours, 48 minutes
Fear the Walking Dead (through season 2): 903 minutes/15 hours, 3 minutes
Total: 5,343 minutes/3 days, 17 hours, 51 minutes
1 These films include a credit indicating that each was "Based on a screenplay by George A. Romero", and had Romero's blessing.
2 These films could be considered official sequels, but usually aren't. The screenplay of the original Night of the Living Dead was a joint effort between George A. Romero and John A. Russo. They disagreed over how to handle sequels for the film, and ultimately agreed to go their separate ways, with Romero's sequels having "________ of the Dead" titles, and Russo's sequels having "________ of the Living Dead" titles. Thus, both Romero's sequels and Russo's sequels are technically true sequels to the original film, and are made by/under the aegis of the creators behind the first movie. In the universe in which Russo's series takes place, however, Night of the Living Dead is a film based on real events.
3 Includes a performance by Judith O'Dea, who played Barbra in the original Night of the Living Dead, lending this movie a slightly higher degree of continuity than the others in this section.
4 Runtimes for these two films are estimates, as no official information is available.
The Resident Evil films add up to 8 hours 1 minute. Funnily enough, they're all basically the same length (between 1:34 and 1:40 long). The Romero films (I counted 7 in the " of the dead" films) add up to 10:36.
Technically speaking, however, I'm not sure that all the Romero films are "Apocalyptic" in nature, since at least some of them occurred before law & order broke down. I'm unsure, so I lumped my answer simply as "zombie" films.
Side bite: Army of Darkness takes Ash back to 1300AD. I don't know whether that qualifies as a 600 year long Apocalypse or not. Possibly, this is a different question altogether, but seemed a fun notion.