Went on a second date with this guy. Hard to tell if he is shy. He said that he'd call to get together again. That was Thurs night, now it is Monday. How long do I wait. I don't feel comfortable calling him.

Keep waiting. I know that answer is hard to handle but if you are not texting or calling it makes him wonder what you are doing. If he is really interested you'll hear from him. In the mean time keep doing the single thing. There are many guys out there. Find the one that's right for you. Don't focus on being what they want.

You continue on with your life. If he calls he calls. If he doesn't he was not interested. Or you can call him if you liked him.
There are no rules.