Depends on the airplane, but most airplanes are designed glide with no engine (Edit: Technical explanation below).
This is a British Airways Boeing 747. This particular airplane had all four engines shut down mid-flight when it flew into volcanic ashes over Indonesia. It landed safely with zero fatalities.
This is how a jet engine looks like after flying into a cloud of ash:
Mid-flight is probably the safest time to have engine failure, if there is such a thing as safe engine failure. It is a lot more dangerous during takeoff and landing because the airplane might not have enough speed or altitude to glide safely. There are far worse situations to be in than having no engine.
EDIT: Technical explanation
What determines the distance and how fast an airplane descends without engine is a parameter called glide ratio. The glide ratio is also equivalent to another parameter called lift-to-drag ratio (L/D), which is exactly what it says.
Glide ratio = (distance travelled forward)/(altitude lost)
Glideratio=Lift/Drag=CL/CDGlideratio=Lift/Drag=CL/CD
So a glide ratio of 20 means that the airplane would have travelled forward by 20 meters/feet for every 1 meter/foot of altitude lost. L/D itself can be complicated, but for many airplanes it’s related to the Aspect Ratio of the wing (AR), which is:
AR=(wingspan)2/(wingarea)=b2/SAR=(wingspan)2/(wingarea)=b2/S
The larger your aspect ratio (i.e. the longer and more slender your wing is), the larger your glide ratio will be.
To give an idea what some realistic L/D numbers are:
Glider/sailplane: Around 40 — this is probably as good as it gets because they are designed to glide for a very long time.
U-2 spy plane: 26 — pretty close to being a glider.
Airliners (at cruise): 14–20, depending on type
Cessna 172: About 11 — very common general aviation airplane.
Concorde: About 4 during takeoff/landing. This is pretty obvious when you look at the wings; they’re short and stubby compared to other airliners.
Space shuttle: 2 — perhaps ironically, the spaceplane that’s designed to glide to the Earth is actually a terrible glider. Delta wings like the ones used on the space shuttle and Concorde are not meant for gliding long distances. In this case, there are many other design factors that led to the space shuttle being what it was, so everything considered, this was the best they could come up with.
Also, the equation above is less straightforward with delta wings because of the unique way they generate lift. Delta wings, however, are great for supersonic flight, which is why they’re often found in fighter jets.