Yas Island's own multi-park page puts 6 to 7 hours on each park, just to work through its rides and experiences. That is the number to plan around, before you look at a single ride.
Three of the four — Ferrari World, Warner Bros. World and Yas Waterworld — also run a strict policy against stepping out and back in, so once you've left for the day, that park is done. Squeeze two parks into one day and you will rush both.
Doing two parks or more? Yas Island sells a Multi-Park Experience: pick 2, 3 or all 4 parks on one booking; the clock starts at your first park and runs for 6 days to fit in the others. Yas Island's own estimate puts the saving at AED 200 to 700 against buying each park separately, scaled to how many you add.
We sell single-park tickets to all four. For a one-park day, the pair of prices on each card above tells you what you need; for two or more, add up our tickets and weigh that total against the official pass before you commit to either.