Personally I would take as many days as I can get. My ideal during 'the non-Holiday seasons is to get in 5 full days and I would want at least one more day in December. The Holidays are an amazing time at Disneyland and there is a lot to take in that is specific to that time of the year. If you can swing it financially without breaking the bank I would recommend going for that extra day. How many days are you used to doing in February/March?
I agree with you, Jessica! If it were to border on being overkill or tiresome for some folks, maybe adding in some extra breaks during the day to kind of mix things up would be good. But with 5 Santas around DLR, all the cute details in the Round-Up, the glorious ambiance of the GCH during the season (and I know you feel the same way about it that I do!), the different window displays and themed decorations & music in each land, the merchandise, the different holiday treats, the snow on Main Street, the parade & fireworks, etc., time can fly by pretty quickly.
I guess it boils down to how much you love Christmas in general and what things you like to do. I suppose that if someone is not a big Christmas person at all, then the idea of 6 full days (or even 5 full days) wrapped up in the middle of it could be misery. Some people will probably have had their fill of it after one day!

If you love to take photos of all kinds of details and things around the parks, Christmas time is a great season to do that. If you're not a big photographer of the little nuances around the parks, maybe 6 days would be too much.
Personally, like Jessica, the more days the better for me. I will examine every inch of the Reindeer Round-Up, ToonTown, New Orleans Square, Downtown Disney, etc., look at every window display, go in every shop I can manage, sit in the GCH lobby stalking Santa and the carolers for hours, sit on Main Street as the different vehicle drivers honk out some holiday tunes on their horns, eat gingerbread cookies and peppermint ice cream, find every tree and wreath in DLR

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, ride HMH and IASW Holiday repeatedly, etc. And don't forget the teeny decorations on the Storybook Land ride and the giant ornaments in A Bug's Land!
But...certainly a 5-day trip will be great too! It's not like 5 days would be awful and 6 days would be wonderful. It's just that, if you love the season in general and the overall Disney attention to detail, the more days the better in December!
