I keep mine in my beer fridge. Not all of them are viable after six months of refrigeration, but there are enough viable seeds to make it worth it. I have read about others freezing seeds, but I can not help but think this only works with certain plant types. But then, some seeds, not sure which ones, actually have a better germination rate if they are frozen, since it sort of replicates a natural winter cycle for them.

I don't have a time limit for mine. I pre-sprout 10 seeds in a coffee filter/baggie and if they sprout o.k., I plant them. Some don't sprout and those I throw out, but most of the time they do sprout.

This link includes a chart with the approximate storage life for many kinds of vegetable seeds (scroll down for the chart). There's a pretty big range, with tomatoes, eggplants and squash lasting for several years and onions only for a year or two. Based on my personal experience, if you store your seed carefully, the numbers in the chart are conservative. Well-stored tomato, eggplant, and squash seeds can sprout (with lower germination rates) for ten years or more. In fact, one of the tomatoes in my garden this year came from 2004 seed.

So, except for onions, there's no reason to throw out seed each year. (Even with onions, I usually get good germination the second year, and sometimes beyond.)

When you buy seed or get it in trade, sometimes you don't know how old the seed was when you got it.

When I have seed that I'm concerned about (old or unknown age), I do a germination test with ten seeds to see if it is still viable. I test far enough in advance of planting time that I can order replacement seed if needed. Sometimes I only have just enough seed to plant, so I can't spare ten seeds for a germination test. All I can do is plant and hope...and have a backup plan in case they don't come up!

The real fun is when you start saving and trading seed. It's fun, and you can get a lot of really interesting seeds by trading with folks - varieties that are difficult or impossible to find commercially.

Here is a link that might be useful: Seed storage life chart

I sort through my seeds every year, but I often keep most several years or more - some last quite a long time (like tomatoes)

They are mostly stored in large tins & jars w/ silica gel packets, but I always have a number of cups & dishes & bags w/ seeds drying in them - some get left out longer than I'd wish

They are in my house, near the A/C - IOW, a cool, dry place

I read that the bigger the seed, the longer they last. And that Parsley is only good for one year. Check out Round Robin Exchange. There is an awesome exchange in progress posted by The Garden Geeks that covers alot of sites.