Our old playdough was more than a year old and starting to smell "off" so I decided to make some more a couple of weeks ago. I was surprised how well it kept. It's been in an airtight container in a cool, dark place, but not refrigerated or anything.
Maria, Erika and Elizabeth helped me make the dough, so we ended up with lots of different colours. The new batch is kind of sticky - the Internet says this can be easily fixed by adding more (gluten-free) flour, but I haven't actually tried this yet! Elizabeth is enjoying it anyway, making oodles of pretend cookies and lecturing me if I pretend I'm actually going to eat them. "Mommy-dear! Those aren't tasty!", she says. I'm trying to not care that the different colours of dough are slowing getting mashed together.