Well,of course it depends on the way you consume manga.If you buy the hard copies,you are much less likely to lose interest or stop by chance,especially because you can always buy complete series.Though I have to admit that,as most series are ongoing and only available online,even I tend to forget about several series.Reading manga is different from turning on an iPod or cracking open a book. The main difference is that we have to wait for releases as they come out, and most of the time we read manga online. Unlike other hobbies, there are a lot of steps that need to be taken in order for us to be entertained; the fact that it's a medium that is specific to the Japanese language plays a role.
EDIT: I prefer to read novels in one sitting, but I can't do that with manga since I have to wait chapter by chapter for an on-going story to be completed. Personally I read prose fiction faster than manga for some strange reason.

) As much as possibe I try my best to read on thinking that maybe it'll get better some chapters later. But when I keep yawning and get very sleepy reading it, I know I've reached my limit.