I don't think any more than 2 hours. Maybe even less. I think we were there for an hour but it was very busy. I only have 1 kid tho not sure how much longer you would need with 4.
If the kids don't know about Sea Life just stick with LL, unless there are fish fanatics in the bunch.
We haven't been to Sea Life yet but do the Atlanta Aquarium in about 2 hours, even when it is packed so i would think something close to that would sound right. My DS5 at the time really wanted to do sea life when we were there. I wish they left it open a bit later.