Shelf Life Of Yeast Starters?

[King Brown]

After reading Batz' Interesting thread on making yeast starters from Wyeast, I would like to try this instead of using the whole package on my next brew. The only thing I'm not clear on is how long will one of these starters keep in the fridge before it is no good? I doubt I will be making 6 batches of witbier in the next month...

Even if I don't end up using all the starters being able to use one packet on several batches will be a welcome money saver!