When do pilots disconnect the autopilot on an ILS approach?
The decision to disconnect the autopilot is made by the pilot and can be made at any point as long as the operational limitations of the aircraft are satisfied (according to the type of ILS). The pilot also has to satisfy the applicable regulations and company SOPs (as @Sami already pointed out). Note that this depends on the pilot decision too- the pilot can fly the missed approach in autopilot if he decides so.
Usually, the aircrafts have minimum altitudes below which the autopilot has to be disconnected, which are given in the aircraft operating limitaions. For example, for Airbus A340,

CATEGORY II
If the crew performs an automatic approach without autoland, the autopilot must be disengaged no later than at 80 feet.

For Boeing 777:

Autopilot/Flight Director System
The autopilot must be disengaged before the airplane descends more than 50 feet below the MDA unless it is coupled to an ILS glideslope and localizer or in the go–around mode.
Without LAND 2 or LAND 3 annunciated, the autopilot must be disengaged
below 200 feet AGL.

For Boeing 787:

Autopilot/Flight Director System
Without LAND 2 or LAND 3 annunciated, the autopilot must be disengaged
below 100 feet AGL.

As an example in GA aircraft, for Cessna C172 G1000:

The autopilot must be disengaged below 200 feet AGL during approach operations..

As you can see, there is not much difference.