It depends if you are doing funtown and splashtown. If you want to do both families start at splashtown usually at 10 and then head to funtown after since it's open longer. Splashtown can be 4 hours -8 hours depending on your family. Funtown is probably 3 to 5 hours for families that haven't been. Others do funtown first and grt to splashtown by 2 or 3 and stay until it closes at 6
Just an additional note to consider, when you get your wristband you can enter and leave the park at will for the rest of the day(I know this is true for the combination pass but I assume it applies to all passes).so you can go to a local restaurant if you want to save some money or you can visit another local attraction if you need a break
For families like mine driving upwards of 2 hours just to get to Funtown/Splashtown I would recommend staying until close, you have to get your money's worth. Also, factoring in time spent waiting in lines at the rides as well as time to eat you really want to make it a full day thing. I know when I go to an amusement park I like to get there as soon as the gates open and we stay the whole time until they close as we don't go often.
Depends on the family, but most will stay all day. Splashtown USA does close earlier than Funtown typically.
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