Reserve as soon as you know which dates you'll be in Orlando, especially if you want to swim with a dolphin. These slots fill up fast.
I'm sure there must be some limit to how far in advance you can book, but the web site says nothing about this. If you know you're going to be there on a definitie date two years from now, I'd suggest booking now, just in case. You can cancel at no charge up to 45 days before the date of your ressie.