If all engines fail in a Boeing, how heavy are the flight controls?
A 747 doesn't have a ram air turbine (RAT), but it has an APU that can power the hydraulics. If that can't be started, a 747 can still function using the hydraulic pressure from each engine's windmilling. Computer based training video here.

Trivia: if all hydraulics fail, but engines are running, a 747 is a dead stick, but can be steered using the engines. A complete loss of engines and hydraulics is never in the emergency procedures of a heavy airliner.

In a 767, assuming the APU also won't start, the RAT will provide hydraulic pressure. Controls will be heavier (but not impossible) the slower the plane is.



Source: 767 QRH, 13.14



767 RAT speed below 20,000 feet is >200 IAS, any slower, hydraulic pressure will be lower.

A 787's artificial feel system (Elevator Variable Feel) will revert to direct mode, feel will be based on flap position, and during approach feel forces will be reduced to make the landing easier.



Source: 787 FCOM, 9.20.12

Trivia: a 787's RAT can supply 5000 psi of hydraulic pressure.

In a 737, the crew will use manual reversion mode, for roll and pitch it's control tabs, like an MD-80, heavy but not impossible.



For all types, it will be almost impossible to find data on the exact amount of force needed. Also for all types, if no APU or RAT, windmilling can still provide some hydraulic pressure.

Same for the pilot feedback, as very few flew all-engine-out heavies. And adrenaline rush would almost certainly skew their post-flight judgement.