I went yesterday, the bus from Windsor which is near the Windsor castle is a shuttle bus which costs £10 for a return family ticket and takes about 20ish minutes.
The buses are quite often, around one every 20 mins or so and when you get to Legoland the stop has a timetable which you can look at it in regards to what time the buses leave when you want go home.
My tip is to take a photo of the timetable on your phone so you can glance at it when you are wanting to leave the park.
Had a wonderful day yesterday although the weather was a little up and down!
Food is very expensive there which is why I'm glad i took food with me. It was £13 each for a course for an adult and £6.95 for a child at the restaurant near the park 
we have 4 dc, so we walked straight back out after seeing how much the food cost and the all you can eat buffet was similar prices to that above each.
Although i did treat the dc to some chocolate coated waffles, but beware they are v messy.
Got to go on quite a few rides, the only one i would say was a waste of time was the Ferris wheel type ride near the Egyptian part of the park-thought the queue was short, but it took ages to load each family and stopped right at the top (gasp) only to go round twice, what a waste of time.
Atlantis is good, the kids loved it and all ages can go on it.
The diggers are great but there was a long queue for that but was worth it to see the dc faces.
There was a train thing that went in a circle, although looks harmless, it goes pretty fast, I'm surprised my 2yo liked it!-i had to go on it 3 times with 3 of my dc as they allowed me to do a child swap.
The models are excellent although a few of them could do with a revamp, and a lick of paint/new bricks.
Toilets, wonderfully clean and even have baby seats in a few of them to stap baby's into so parents hands free to use the loo.
They do a buggy hire for £8, wished i had done it as it would have saved 4yo from needing to nap in 2yo buggy after a long day as they do doubles too!
Gift shop is expensive, £12 for a basic cap , there was even a dress which wasn't anything special costing £69.00!!! I allowed the dc £10 each and even then there wasn't much they could afford so be prepared to part with lots of mulla.
There is a new duplo splash towers being built for may, it looks brilliant!
We and dc had a fun time, just wish we didn't live so far from Windsor. We left at 4, and didn't arrive home until just before 8:30, i ordered dinner for them when we got home and then they crashed out bless them.