I haven't been yet, but from what I've read, it can be done in a day. However, you might miss some sections (like walking through the gardens). The other thing, I've been to Disney World several times over President's Weekend and vowed not to do it again. It is usually at the same time as the Daytona 500 Nascar race, so you will get people from that coming over to Orlando. Also look to see when Mardi Grad falls. One year it was around that time and since New Orleans schools were out, there were lots of people from there at DW.
It also looks like
Legoland isn't designed like DW for lines (no Fastpasses, line queues not made for masses of people, etc). Wait times when it was empty were around 10 minutes for some rides, so imagine what they will be like when it is busier. Finally, don't forget that if you kids are like mine, shopping for Legos will be high on their list! That will chew up some time.
And in case you didn't know, there is the VIP rewards program (free). I highly suggest signing up if you buy lots of Legos. You earn points for every dollar spent on Legos through their stores and online (not at places like Toys R Us, Target,
Walmart unfortunately). I save the big purchases for the
Lego stores/online as it doesn't take much to earn $10 in rewards. Right now until the 31 they are offereing a free gift with any purchase of $99 (VIP members only), double reward points AND free shipping. It was a lot cheaper to buy some Christmas presents online than buying them at the park!
Hope this helps! We are looking to visit next month or January. We are also thinking of making the hour run over to Clearwater to see Winter from Dolphin Tale (daughter is a huge animal lover and has seen the movie 3 times now).