i didn't rtfa too carefully, but from what i've heard, all newly-hired developers at Facebook go through a 6-week 'boot camp' where they fix bugs and do maintenance to get up to speed with the code base ...
Yeah. Six weeks is barely enough time to pick up on on all the architectural and subtle design decisions in the code base. You still probably won't be really comfortable making big changes for at least another 6 weeks after that with any significant code base. Maintenance programming implies deep understanding of the existing code really.
http://articles.latimes.com/2010/aug/01/business/la-fi-faceb...