Watch the vocab here.
- A strut includes the large spring that is attached to the frame
at one end and the wheel hub assembly at the other end. It's job is
to allow the wheel to move up and down freely over uneven road
- The picture you posted is of a shock absorber. It's job is to quickly slow down the up and down motion in the strut to keep the vehicle stable.
Modern vehicles have both as this provides the best ride comfort and vehicle stability while braking, accelerating, cornering, riding over bumps and potholes, as well as vehicle oscillations induced by the road, the air resistance and driver inputs.
Shock absorbers should return to normal "very soon." It's not the time that is critical, it's the strength that they use.
- If the shock absorber has lost some of it's "charge" (some are filled
with gas, some are filled with liquid), it will loose some of it's strength to return to it's normal length. If it's completely lost it's charge, it won't return at all. These things develop leaks over time.
- If the valve system inside the shock absorber gets clogged or damaged, the absorber will loose some or all of it's strength as well.
A shock absorber loosing it's strength will take longer to return to normal, since there is deliberate internal friction part of the "tuning" of the absorber for the weight and type of vehicle it is attached to and the ride smootheness it is supposed to provide. As you noticed while shooting your video, the new strut was a lot harder to compress than the old one.