I personally hate to spend any less time in the parks than 3 days, but have been forced to do so and I am sure I will do again. My upcoming trip in december I will be arriving friday morning and going to the park that afternoon, then having dinner in DTD before heading back to the park for the night. On saturday, I won't even be going to DLR, I'll be in hollywood. Sunday though, the entire day will be about Disney! I would like to have more time in the parks, but I cannot allow it at this time.
When I go in Jan for my birthday, I will be going for the same amount of days, but the entire time will be spent at the resort. This is how I like to do it. I just went on a five day vacation, and could have done more if I was able to!
It really all depends on your stamina, and love for Disney. I would have to agree with the PP's, that 3 days is a minimum.