Two years ago, I did the front yard. I had about 10 pounds left over. I put it in the freezer in the garage and used it to do half the backyard last year.

The seed seemed to germinate just fine. Though I'm sure there is probably some drop off in germination rates.

I read a rule of thumb somewhere (probably these forums) that you want to keep the combined temperature (F) + relative humidity below 100. So if the temperature is 80 and the relative humidity is 15%, you're okay but getting close. If it's 90 and the humidity is 15%, you'll see the germination rates drop off. Keeping it refrigerated or frozen will certainly help, but if you can't keep it refrigerated, just keeping it relatively cool adn dry will help a lot.