I visited Three Chimneys this summer and I would say the tour lasted about an hour. I booked it online; I am not sure if their process has changed in recent months.
I also took a tour that was offered through the Kentucky Horse Park, which took us to WinStar and a couple of other area farms. The tour guide was fabulous. He had worked in the industry for a number of years and thus had a wealth of knowledge about various horses and races throughout history.
I will be going back to do some book events (new book about the history of Churchill Downs - to benefit Old Friends )this year and cannot wait to visit some other farms, so this thread is very helpful!