To tag on to what Murphy said from some AARs and info from some TCs I know.
While turret down the TC identifies the target and gets the range estimation data and direction to the gunner to set the elevtion and traverse the turret. If the tank has a roof mounted panoramic telescope for the gunner he may also be able to see the target too and engage it while turret down.
While TD, the TC gives the command to move from turret down to hull down and stop. The gunner then makes any fine adjustments to get the crosshairs on the target and fires. As soon as the driver hears the shot go off he reverses back to turret down or relocates to another firing position.
Former TCs have told me it takes about 4 seconds to move from TD to HD and back and 3-4 seconds for the gunner to fire while HD.
The defenders need to be ovewatching in the right direction to react and get a shot off before the target disappear.
At least that how it works in my game.
However in some circumstances the "bound" is too long to make a credible time increment for say reloading a guided missile manually from an unbuttoned vehicle. Probably only an issue for those playing at 1:1 with 1/144 scale or smaller.
Here is a video showing a BMP-2 reloading while not exposing a crew member. link
From what I can find the BMP-1 takes from 7.5 to 10 seconds to reload.
Here is an account from a Russian BMP-1 gunner:
The hatch used to reload the ATGM is hilariously hard to use – I have relatively long arms and I still had to stand up and really stretch myself to put a missile on the rail. Malutka is guided using a joystick that folds under the gunner's seat – it kind of looks like you are playing with yourself while you're aiming the thing. I don't think we ever used the autoloader in training because of the risks.
Wolfhag