We were there Monday and Wednesday last week. Both days we got there around park opening (9:30) and stayed until 2-3pm, so about the same number of hours you are thinking.
We were actually quite surprised that everything was basically walk-on. In 5-6 hours we were able to do all of the rollercoasters (some multiple times), several other attractions, and walk through the entire park.
However, if there were lines or if you wanted to see any shows it really wouldn't be enough time.
Not sure how you're getting your tickets (we got ours for $20 per person with our Discovery Cove package), but I don't think I could pay full price for a day there (especially if we could only go from noon to 6pm).