Aloha MB,
You can drink Ying-yang water any time throughout the day. Generally, let it cool until warm before drinking.
I've used it in a lot of situations. An out-of-state friend called with a migraine where she was going to need to be in bed all day and couldn't see straight. I had her make some Yin-yang water, drink it, and call me back. When she called back, she said "Okay, I can see straight now. What can I do about the rest of this migraine?" I worked on her from a distance, and she was read to go about her daily work.
So really, serve it whenever. Add some honey or lemon (unless the person shouldn't have either), and enjoy the balancing benefits.
Much Love,
Shawn