Personally, I'd say 3-4 days. I know that people can "do"
Disneyland in less time, but usually they're referring to going on the rides. I, like you, like to absorb the park. I usually spend 5 part days there.
DCA can probably be done in a day. It's still a young park, and hasn't had as much time to develop as DL. Do make sure to check out some of the details there though - I'm not sure who they had in charge of theming for it but whoever it was had a love of puns. Example - the acronym for Grizzly River Run is GRR. There are little things like that all around. Make sure you go through the Mission Tortilla Factory tour and get a fresh warm tortilla! How long you'll need at DCA really depends on how long the park is open that day and how big the crowds are.
Disneyland will take 2-3 days, depending on how much lingering you do. I live up in Canada and still manage to get to Disneyland every year (lately). I've been at least a dozen times now, and there are still rides I haven't been on and places in the park I haven't really explored.
Downtown Disney is, well, an outdoor shopping mall.
The hotels are nice. At the Disneyland hotel there's a nice little wine celler under Hook's Pointe where you can get some really, really nice wines. The Grand Californian is absolutely beautiful. They offer (at least used to, possibly still do) a free tour every day called "the art of the craft" and they take you around the property showing you the various details and explaining the theories behind the architecture.
I'm back at DL at Christmas, which I figure will somewhat prepare me for the crowds next summer when I'll be there for the 50th.