Unless you are there in the very peak season (July and Christmas), you should find arriving at opening time will ensure you get to do all the coasters with the minimum of waiting. Kumba is the most popular, closely followed by Gwazi, and then Montu, while Rhino Rally (not a coaster but a great ride) also draws long lines later in the day. So if you get there on time, you should easily be able to do those 4 in the first 90 minutes, and then you can relax a bit and enjoy the park! As Vernon says, 2 days is ideal to take in all the detail and all the shows, but if you are real coaster fiends, you should have all the time you need in a day.