Probably never, I think when people are getting on well, its kinda obvious. I might say it once after the date, but not much more. I might say it in an attempt to get another date if things were kind of borderline just to put her at ease, but its not to get in her pants... really.
Its sorta irrelevant. Generally, its pretty clear when you are having a good time and when you are not. I might say it towards the end, like "I had a great time" sorta thing just so they know I want to have another date with them. But besides that your emotions and reactions should be telling enough if you are having a good time.
I would only say that at the very end of the date, unless we were doing something awesome like waterskiing on the backs of dolphins. It's implied that if I am on date with someone, I enjoy being with them.