Have you been before? What time of year are you thinking of.
In my mind these answers will make a difference. We like at least 3-4 days if we can work it out because we like to take a break mid-day. Having said that, we have been many times before so if we miss an attraction, it is not too big a deal because we know we will be back. If we did not know when we would be back, then our family would do 4-5 days, again because we like that mid-day break time.
Also, if we are going at a really crowded time of year, we plan more time and expect our breaks to be longer because DH hates crowds. We usually arrive at rope drop, stay until just after lunch and come back after dinner.