During vectors to an instrument or visual approach does ATC expect the pilot to maintain 250 KIAS until cleared for the approach or instructed to slow to another speed?
Generally no, we do not expect pilots to stay at 250 until final. That would be very fast.
Of course I expect the pilot to slow to 250 indicated (which will show as 270-280 over the ground) when reaching 10,000 MSL. And then I will expect further speed reductions as the aircraft gets closer to the airport and the pilots run their checklists and begin configuring the aircraft for landing. Where I work now we very rarely have to use speed restrictions, and I would guess that most jet aircraft are doing around 180-220 on the base leg without any instruction from me.
Military fighters will sometimes stay quite fast on the base leg, sometimes 250+ knots over the ground. This is not normal for other aircraft.
When I was in radar training with the FAA, in the simulator sessions the general rule of thumb was to assign 210 knots on the downwind, 190 on base, and 170 on final. These were slower on downwind and base than the aircraft would otherwise fly, and faster on final. But that was only simulation.
At busy airports where there is a constant stream of arrivals it is very common for aircraft to be assigned specific speeds, whether over the frequency or via a published procedure.
The 7110.65 says this (paragraph 5–7–3):
When assigning airspeeds, use the following:
c. To arrival aircraft operating below 10,000 feet:
- Turbojet aircraft:
- Assign a speed not less than 210 knots, except for the aircraft as specified in subparagraph b above [operating below a Class B shelf], or
- Assign a speed not less than 170 knots when the aircraft is within 20 flying miles of the runway threshold.
- Reciprocating and turboprop aircraft:
- Assign a speed not less than 200 knots, or
- Assign a speed not less than 150 knots when the aircraft is within 20 flying miles of the runway threshold.
250 below 10,000 is not the only speed restriction.
From ICAO Annex 11 Chapter 5 Appendix 4:
Speed limitations applicable to both IFR and VFR aircraft:
— 250 knots below 10 000 ft ASL; and
— 200 knots below 3 000 ft AGL within 10 NM of a controlled airport.
So unless instructed otherwise, or unless the assigned STAR says otherwise, ATC expects 200 KIAS or less near the airport.
Since your question suggests an jetliner, keep in mind the flap extension schedule. Typically the pilot will have selected the first or second flap extension before the final approach path interception, which will nicely match the expected speed. See: What is the flap extension schedule on approach for an airliner?