How long should water come out of a radiator whilst I bleed it, if the radiator is isolated?
It could be that the 'stat valve or the lockshield valve isn't closing properly (it's not uncommon).

As to how long water will come out, that will depend on the size of the radiator and how fast the water is draining.

If the radiator doesn't have a drain point, I'd put a bowl or tray (whatever will fit under the valve) and remove one of the valves from the radiator. You'll soon see if that valve is letting water past when it's closed - you'll have water coming from the valve as well as the radiator. If water slows to a trickle, it's rpobably coming from the otehr valve. You could then take it off and check.

Note though, the water might be a sludgy black colour, so you might want to avoid getting it on the carpet.

If one of the valves is faulty, the options left are to either drain the whole system down or to freeze the pipes leading to the valves.