Long Delays in Bitcoin Transfers: At what point can you assume the transaction will be canceled?

I have been trying to send BTC to another exchange from Coinbase for 11 hours now. Coinbase says that it can take up to 24 hours for them to cancel a transaction, because I assume it was never included on the blockchain. They say "Sometimes there is a high volume of digital currency being sent globally, and there are more transactions than there is space available in each new block to include the transaction." The rest is here: https://support.coinbase.com/customer/en/portal/articles/593836-why-is-my-transaction-pending-?b_id=13521

Are transactions/transfers dropped as soon as they "fail" to be included in the blockchain. Why is the response time back so poor?

Practical Question: If you've had a transaction "pending" for X hours, when should you assume the transaction will not go through in your experience and the BTC will be returned? This particular transfer time is obviously unacceptable and part of the problem that Bitcoin is having in scaling. ;) I'm sure it's solvable, but there is work for the Bitcoin network to do! Thanks.