Longest unconfirmed transaction? I recently tried to make a transaction with coinbase. In a hurry I put no fee though. It's still unconfirmed and blockchain.info doesn't recognize my hash. But my btc qt wallet is debited. My question is what is the longest unconfirmed transaction you've heard of? Mine is going on 16 days. I'm curious because I would still like the transaction to still go through.
A transaction could go unconfirmed indefinitely. 

The order in which transactions are confirmed is decided by miners, and generally they will be incentivized to include only the highest fee-rate transactions, to maximize their revenue. 

Even a low fee-rate transaction may eventually confirm, once the mempool clears out. But there is no guarantee of this, if the mempool is full for an extended period of time, many nodes may drop the lowest fee-rate transactions from their mempool, to free up memory resources (otherwise, you could DoS attack the network by publishing infinite low-feerate transactions). 

Ultimately, once you publish a transaction to the network, you cannot be certain if or when it will be confirmed, until it is. By paying a high fee-rate, you can help increase the chances you will see a quick confirmation. On the other hand, a low fee-rate transaction may never be confirmed, as eventually all nodes may 'forget' about it. 

If you have been waiting a long time for a transaction to be confirmed, you could publish a new transaction that spends the same output, but with a higher fee. You may have to force your wallet to forget the old transaction so that it will craft a new one (eg bitcoin-cli abandontransaction "txid" when using bitcoin-core), but not all wallets will include this functionality.