Wow... interesting question. Seriously, I think it's all up to the readers. Sure I get a little antsy when I'm reading a really good story, and there's no update from them for a while, and I do have to do a little backtrack to get myself back in tune with the story, but here's how I look at it; if the story's good, and the reader is interested, they'll come back for more when it's there for them. They might have to see where they last left off, but I've come to realize when a story captures you, it's hard for you to forget what it was about. That's how it was with my story "Satan's Playground", I posted it at a completely different site, came back here to post it and try to complete it and someone from Vaffel found it here was all like "I remember reading this story a long time ago on another site, I loved this story and I'm glad there's an update."
So like I said, it's all up to the reader. I've been that way with many stories that are incomplete but have slow updates (not mentioning any names Mare, Mers and Julie!! Lol.) I do my backtracking, and continue on with the update. Is it a hassle? No, because I'm interested in the story, obviously the story captivated me when I began reading it. Sure, people tend to lose interest if it's not updated for the longest, but it's funny because as soon as there is an update, people do come swarming back to read it, but everyone's different. I've lost a few of my readers, but the thing is they have to understand that authors do have lives to live, things to do... maybe they're just too busy to get around to their stories, or maybe they're suffering from writer's block, and that's something pretty serious with authors, lol. I mean, it takes a while for an author to complete a good novel, and that's how I look at fanfiction. If anything, updating stories is a task for me personally because I have to do MAJOR backtracking to see where I left off and how the story was suppose to go anyway, lol.