When we were there in May the Lifeguards went off duty at 8pm but the pool stayed open. People were swimming at all hours. I don't believe the pool ever closes. The Lifeguards do announce when they are leaving and what to do in an emergency.
...it depends on the time of year you are going. If it is later in the year (Fall/XMas time) then the pool usually closes at 6PM. Otherwise, it usually stays open till 8PM. But, you may swim after these hours "at your own risk". The Trout Bar closes early, though !
When we were at the WL this past Feb. the lifeguards left at 10:00 pm. As mentioned, they shut the slide down when they left. People were in the pool at all hours.