Studio B/Top Draw have shown they can animate 26 episodes in around nine months or 39 weeks (and they could potentially do even more in that timeframe, although they're doing less for MLP:FIM season 3). On average, that's about a week and a half per-episode. But that would be if everyone piled onto one episode. It works out better if they have different fractions of their manpower work on different episodes for longer periods of time. That multiplies the time that an episode is in development. But even so, "weeks" is probably the right time scale for an episode.
Voice actors can sometimes bang out a year's worth of lines in a few days. It's not a full-time job unless you can get big roles in several different shows at the same time. Not sure about the writing/storyboarding/music, but the final editing doesn't take very long.
If you follow one episode from concept to completion, it can take almost a year for the longest ones, but that's because it's going from step-to-step and waiting in lines, and it kind of assumes that other people are inactive while it's being worked on.
Animation is pretty amazing, but when you've got a well funded, organized, and equipped army of talented professionals, they can really get the job done.