They close at 6 pm. You can rent them for longer than 30 minutes if you want. The cost is pro-rated, so you will pay for exactly what you were out, no worries about paying for 60 minutes when you were only out 45. It was about $20 per half hour last fall, not sure what the current price is.
Make sure you have a watch - when we went last time DH took one kid and I took another but I had taken my watch off for some stupid reason so I had no idea how long we had been out. He went back after 30 minutes and I was out for an hour without realizing it!
We rented them at the WL, but that's the same waterways as the Poly. 30 minutes is certainly enough to get a feel for the boats and have a fun ride, but to truly explore all over, which was my reason for renting them here, it takes closer to 45 min or an hour. WL, Contemporary, Poly, GF (including Wedding Pavillion) and MK are on that one lake. It's amazing to see these places from the water. While we were out, we actually saw a wedding at the Poly up on that high bluff. It's terrific and we all loved it, especially the kids, who were 11 and 15 at the time.