I would plan about two hours. There is a small museum there too. At the end you can have a cold drink too, which is another way you can help support the cause. I don't know how he stays in business, with only charging $10 per head.
We usually spend a couple of hours at Parotte Ville, and usually bring a small bag of fruit for the birds. Then we buy a beer on the way out. It's a relaxing visit for sure.
We usually send a couple hours and finish it with buying ice cream bars at their small snack bar. My wife would spend more time as she misses our parrot at home but the others had enough.