The Persistence of Memory
Because these stories are serialized, I think a larger passage of time in an event makes it more interesting. Characters are not just reacting with the situation at hand, solving threat after threat, without rest to ponder their actions. If they take time to regroup, to consider their positions, or hatch plans, and so on, it gives the event in itself more validity. It means this is not just one bad day for the characters involved, it’s an important event in their lives, and they must adjust accordingly. It means that this event will stay in the character’s history for a longer time (not counting retcons, of course), because it took a considerable part of his or her life and altered it. I think an event that did this almost to perfection was Annihilation, which truly felt like a war as opposed to a single battle. A war with fronts, with changing tides, with strategies that took time to develop, where you could see the wear and tear in the characters beyond some tattered clothes: they were tired of the conflict and wanted to end it as soon as possible. It took a toll on the characters as they grew more desperate, it forced them to change, to reconsider their alliances. On top of all that, Annihilation actually kept track of the passage of time, with handy captions reminding you how much it had passed since the first day of the event. Any character can have one really bad day and just forget about it, but weeks, months, or years worth of conflict? You can be sure that's going to stay in the memory of a character and the reading public for the foreseeable time.
A Week In The Life
Interestingly enough, companies have been experiencing with shorter events. For example, Siege was only four months long, half as long as other events, though it still suffered from the same defects as Blackest Night, where it is all one contiguous series of events. The battle was literally shorter than a day, and it all takes place within the book, but we barely see any of the fallout in there. An even more interesting experiment was War of the Supermen, dubbed “The 100 Minute War” because that’s how much time passed in the story, which was released on a weekly schedule. While I can’t vouch for the quality of the story, I think it was a smart move to release it in that schedules. Can you imagine what it would have been to read 100 minutes worth of fictional events over the span of four months?
At the same time, give too much leg room, and the tie-ins lose relevance and their ties to the event become tenuous. Final Crisis did a great job in conveying the passage of time, and it helped assert the seriousness of the situation (Darkseid was in power for some time before being taken down). The problem stemmed from the tie-ins which, because of the scope of the story, were basically allowed to tell stories that barely connected with the main event.