Lindsay's suggestion is good, I'd add to keep in mind that if your media contains l-glutamine, the l-glutamine breaks down faster than the medium itself. If I recall correctly, the "shelf-life" of l-glutamine in media can be as quick as 3 weeks.
One way around this is to make the sterile media without l-glutamine, and only add the l-glut when ready to use.
4 weeks is ideal but you can still use it for washing of your samples if you are really concerned about wasting your resources. The reason for that has explained by Joseph in detail.