Can all helicopters hover, and how long does it take to "stop"?
As noted in another answer, all helicopters can hover, but a so-called "high hover" (out of ground effect or especially at operational altitude) is a more difficult maneuver, requiring more power than a ground hover, and being harder to maintain (because reference points are much further away).

Helicopters generate more lift for the same power when in forward flight, and transitioning from forward to hover requires a well-controlled combination of adding power, adjusting collective, and maneuvering cyclic (in both axes) and anti-rotation controls -- which is to say it's much harder than simply flying forward at low speed.  In general, once aloft and flying forward, it's much easier to continue forward flight (it's also safer, since in the case of a failure, autorotation works better if you already have some forward speed).

Hovering on instruments is even more difficult than a high hover -- so if visibility is a problem, it's generally far better to continue forward flight than to attempt a high hover.