Using myself as an example: I started 2009 aiming for 1500 calories a day, and reduced to 1400 in mid 2010. Based on my rate of loss, I expect to maintain at around 1750 calories a day.
This past week, I ate in the neighborhood of 2600 calories on Tuesday, 3000 each on Wednesday and Thursday. That's approximately 3350 calories above my estimated maintenance level, for a gain of about a pound, which agrees with what my scale is telling me.
Now, I noticeably overate, but I did not binge, and I wasn't full to the point of bursting--it was just normalish holiday eating. The illustration I'm trying to make is that it's not at all hard (for me, anyway), to go 1000-1500 calories *over* maintenance on a given day, but it's nearly impossible, or at least unhealthy, to eat 1000-1500 calories *under* maintenance in a day.
That's one possible answer to your question--it potentially can take very little time to gain weight, and a long time to lose that same weight, because it's easier to create a surplus than a deficit.
For me, the moral of the story is that, even when I know I'm overeating, I try to monitor the calories for two reasons: (1) makes it less likely I'll go completely overboard and (2) gives me an idea of how many calories to try to "pay back".