Expiration time of transactions with insufficient transaction fee?

Hi there!

Question: If a Bitcoin transaction is sent, and the transaction fee is zero (or not high enough), the transaction will not be forwarded through the network, because other transactions have priority and took up all allowed space. So, the transaction will not go out. I'm wondering how long it is necessary to wait, before the transaction expires? Does it ever expire, or is there a risk that some far point in the future your transaction could suddenly go through?

Is it possible to resubmit the same transaction (spending the same coins twice), but the second submission has a higher transaction fee, so it overrides the first transaction and effectively cancels it? Or does the Bitcoin protocol not support this? Or is there something I'm missing?

Thanks!

Josh