I have seen many people recommend no longer than 2 weeks, but I once left a 2L starter for a month and had a problem-free fermentation.
I don't see how refrigerating a fresh starter is that different from buying a new tube of liquid yeast. Granted Wyeast/White Labs probably treat it in a very specific and healthy manner, oxygen free, nutrient-enriched, etc., but I'd say you're fine for a few weeks at least.
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