Been paying a lot of attention to visitor happiness in my current zoo, and I've now seen maybe a dozen instances of unhappy people leaving with the message being "I wish I could have seen..." for animals I did have in my zoo. I then checked the stats of these visitors, and every one of them showed a time in zoo of between 57 and 59 minutes. I then paid attention to other, happy visitors leaving the zoo and they also showed time just shy of 60 minutes.
My conclusion is that visitors will leave the zoo just a bit before hitting 60 minutes, regardless of if they have seen everything they wanted to or not and for some, not seeing the animals they wanted to because they ran out of time makes them unhappy. Doesn't matter that the animals are there and can be seen, just matters that they didn't have time to see them. Conclusion: The larger your zoo, the bigger disadvantage you will be at for this particular metric.
If this is accurate, Frontier really needs to take a look at refunds related to this. That being said, these visitors are STILL profitable for your zoo. When I looked at the unhappy ones, they all spent a decent amount of money before leaving. And, keep in mind that a refund is NOT a loss of money. It's merely a net zero on the ticket price. It may show up as a negative in the finance report but keep in mind that every one of those refund dollars was also a positive in the income-zoo ticket row at some point.
I should mention, I show 0 refunds for the past 2 years in my current zoo where I was doing this, so there's more that goes into the visitors deciding they want a refund than just happiness + if they saw the animals they wanted, because even the unhappy ones who didn't see what they wanted to who were leaving my zoo were not asking for a refund.