Glutamine "works", but not for building muscle like creatine. Think of glutamine as a muscle preservative. The time to be taking glutamine isn't when you're trying to build muscle, but when you are trying to keep what you have.
For instance, many people don't like to eat a few hours before bed, especially nothing heavy like protein. This would be a good time to take glutamine, right before bed. When you awake in a fasted state, this would be a good time to take glutamine. Any time you are dieting or fasting, especially low protein diets, glutamine is a good thing.
Another good time for glutamine is at the end of a cycle. This is for two reasons, you'll be able to start eating less protein to keep from getting fat while still preserving your muscle, and second because glutamine inhiibits cortisol, and the decreased ratio of testosterone to cortisol is believed to be the cause of lost muscle mass at the end of a cycle.
The time not to be taking glutamine is when you are consuming protein and trying to build muscle. Any large dose of one amino acid has the side effect of inhibiting the absorption of all others, so don't take glutamine at the same time you consume protein, or better yet until the protien is fully digested.