While I wouldn't recommend actually doing it, it can definitely be done. You can totally knit a sweater in a day.
It just might be sweater that is only big enough to fit a six-month old baby, but whatever. It's a sweater.
Project: Baby Pullover by Knitting Pure & Simple--one of this week's 30 Projects for $30 (And Under!) at the shop. (I am waaaaay behind this week.)
Started & Finished: Wednesday, May 11th, 2011.
Yarn: Malabrigo Rios in Playa--1 full skein.
Needles: Addi Turbo US 8 & 5 16" circular, plus Brittany dpns in US 8 & 4s. ( I didn't have 5s at home.)
Size: 6 months.
Mods: None. I've knit this sweater so many times, I can almost knit it in my sleep. Which would be helpful, because I've got so many things to knit before the weekend, I have a feeling I'll be knitting continuously for the next 96 hours. Sleep, I fear, is not on the schedule.