Pet food quantity - large package or small package?

I buy medium large packages of dry food, and they usually last for about a month. I keep the bag shut, and the smell stays the same, but I have never though about the food loosing it's nutritious value for staying open too long :O Is a month too long? I usually leave the dry food in the bowl for them to eat when they're hungry, does that get bad too? My parents have taught me nothing about food, apparently T.T... I buy the bigger cans of wet food too, and they last for a few days, but I keep them in a closed dish in the fridge, not in the can, and I never leave it outside in their bowls when they're not eating. The treats last for a while too, but they're in shut containers too, so I hope they don't lose their nutritious value.