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In clear weather, when do pilots disconnect the autopilot, if the plane is flying an ILS approach and the autopilot is in "approach" mode (tracking both localizer and glideslope)?
Would the answer differ significantly in different planes, for example Boeing 777 vs Airbus A320?
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,
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.
I am a B777 pilot; on approach I typically disconnect the Autopilot once visual contact is assured and I am inside the Final Approach fix.
If there is any chance of a possible missed approach, (especially with a complicated missed approach procedure) I will leave the AP on until there is virtually no chance of a missed approach due to weather or other conflicting aircraft on the runway or elsewhere.
Most of the pilots I fly with tend to do the same.
On departure, we generally hand fly until about 10,000' to keep up our skills. If the departure procedure is complicated, we will ask for the AP on at 200' so there is no chance of messing up the departure procedure.
In calm wind or under 5 knots on a cat 3 then ride that sucker all the way down and it will disengage when the rear wheels hit the runs way, but keep your thumb on the elevator trim in case it doesn't. In most normal conditions keep it around 50 ft. agl. In low visibility then keep it on until you see the runway lights and feel safe to disengage and land/glide manually. If you start to oscillate before touchdown then turn the A/T off reduce to idle and disengage the Auto Pilot and dip your right rear or left (depending on wind direction) and grab that runway letting the plane settle. If you are cat 2 then disengage no lower than 20 feet agl, so you can adjust your glide and attitude or roll for a softer landing. If you are cat 1 then approximately 50 feet or until you see the lights on the runway in low visibility. Discretion is key here or course, but trust the glideslope in low visibility as long as possible and make sure you have the correct QNH and be ready to mash that TOGA button if something isn't right.
