If you're going at Easter you may find it hasn't warmed up enough by 9am to tempt you to lounge around at a water park - it depends how hardy you are! I'm a real wimp, so it has to be pretty tropical for me to strip off to a swimsuit, but DH would go in in a snow-storm if he felt like it!
That aside, we find that four hours is usually plenty of time for DH and DD2 to have their fun on the slides and shark reef, and for DD1 and me to take a few turns around the lazy river and browse in the shop . (I told you I was a wimp )
How long you want to spend there may depend on what else you have lined up for the day - TL is a beautiful place to spend the whole day if the weather's great and you just want to relax.
We also tend to spend only the mornings at the water parks (but we only ever visited during August so the weather will be a little different from your holiday). We make sure we arive at the water park just before opening time, spend the first houre or so on the slides, then we head to the lazy river to chill out or to the pool for a bit of bobbing about in the waves. By lunchtime the weather is getting too hot and the crowds too busy for us to enjoy ourselves so we tend to leave for an midday break before heading to a theme park.
Weusually arrive at opening and leave about 3-4 O'clock. It can be a pretty tiring day so usually would only plan to go home, shower, relax a little then get ready to go out for a nice meal. An alternative is to use the evening for a trip to the Cirque. I would find anything too late, like a trip to PI @ DTD , a bit too demanding after a fun day in the sun.