However, if you keep an eye on several block explorers like blockchain.info, blockcypher.com, blockexplorer.com, btc.com, blocktrail.com etc... when they no longer show your transaction, it is quite likely that the majority of nodes have dropped your transaction and you should be able to respend the coins.
I'd like to add that it's fine to try to resend before it doesn't show on block explorers. I made transaction in December which remained unconfirmed for over 21 days. It still showed on blocktrail, blockchain.info and a few others that I checked, but Trezor let me create a new transaction where I used a proper fee. Despite the transactions showing, it got broadcast successfully and confirmed in the next block.
I don't believe Electrum tries to rebroadcast automatically - you have to click a button for that.