So essentially, any plane can glide if the need arises. And in situations where all the engines have failed, pilots have to expect the plane to do some gliding. Without the thrust those engines are built to provide, the plane can't help but lose altitude. But how far can a plane glide when it's not designed to be a glider?
Aircrafts whose engines conk out at higher elevations can glide for longer periods of time. This is one of the reasons why Sullenberger and Skiles' Hudson River landing was so impressive; they had to glide their way to safety in a manner of minutes from a pretty low altitude. (Everything happened very fast on U.S. Airways Flight 4951. The plane hit the birds within two minutes of taking off and just three later, the plane was in the Hudson River.)
Obviously, planes come in all shapes and sizes. So if you're flying one, it's important to know your vehicle's "best glide speed." In a nutshell, this is the speed that will let your airplane travel the farthest distance while sacrificing the least amount of altitude.
A related concept is the minimum sink speed, the pace of travel that'll maximize how much time you can spend gliding. Depending on your situation, you may choose to prioritize time over distance or vice versa.
Writing for USA Today in 2013, veteran pilot John Cox stated that a jetliner could probably be expected to glide for around 100 miles (161 kilometers) if all its engines failed 30,000 feet (9,144 meters) above ground level. That's about the distance between Los Angeles and Palm Springs or New York City and Atlantic City. In other words, not very far.
"Having all engines quit in a modern airplane is extremely rare," Cox also noted. That's reassuring.