There is no exact science. If there aren't many popping up, we tend to ignore them. When the trend of OT posts gets too numerous, we run a quick skim through the threads and move a bunch. If your OT post was this week as part of a trend of too many, you'll get swept. If it was an isolated incidence on a mostly on-topic week, it might get left.
Also, some topics are more clearly way off topic than others. For example, a post about the TV show heros, while not technically about comic books is pretty clearly sort of comic book related through the super hero concept. A post about Battlestar Galactica... ok... not very comic booky but a pretty good bet to be of interest to the crowd.
A post about flying model airplanes... water cooler.
Essentially it's a process left to expediency and based on the general feel among the participants and moderators of whether CG is feeling too off topic. We will not be refining that process as it wouldn't be worth the time consumption. Please just utilize common sense and try to keep things generally on topic. Thanks.