So you're after a measurement for the amount of force required to turn the handlebars? Googling's likely to give tightening values for bolts and fasteners.
Inputs that will affect your values:
- Geometry of frame and fork - a steeper headtube would reduce force required to turn away from center, and a slacker headtube would make it harder.
- Fork's effective rake and trail values - the shorter the trail the easier to turn. If the head tube is aimed right at the contact patch that would be the smallest.
- Tyre width - a narrower tyre will have a longer contact patch, of which the tips would be further from the center of rotation against road, requiring more force to turn. A wider tyre would have a rounder contact patch.
- Tyre air pressure - similar to width.
- Tyre load - more weight on the tyre means harder to turn, unless the pressure is raised to match
- Tread depth and condition - Not sure if a smooth tyre would be less or a knobby tyre. I suspect a full knobby would have more flex and allow easier turning. A brand new tyre will be slipperier on the road than a slightly used tyre.
- Bearing tension and quality and cleanliness in the steerer/headset.
Dynamic considerations exist too
How far off-center the wheel already is - turning from 20 to 30 degrees will be different from turning from 0 to 10 degrees.
Speed - Most effort will be needed at a dead stop. A slight forward motion will help immensely in reducing force required.
Speed - gyroscopic forces on the wheel will resist turning as well. Note this is nothing to do with balancing. A heavier rim/tyre will exaggerate this.
Ground angle - the angle you're riding up or down will affect the weight balance
Ground surface - loose material like shingle will move easier.
The main problem is there are so many variables and they all interact. I can't give you a number however with a measuring tape and a spring balance you can get your own numbers for your bikes.
Measure from the center of the steerer tube to the end of your bars. Might be as little as 0.30 metres for a road bike, up to possibly 0.90 metres for a wide MTB.
Attach a spring balance to that point on the bars.
Sit on the bike, and gently pull the spring balance until the bars move. Do your best to notice the steady state value while the bars are turning smoothly, which will be less than the peak load when it first breaks free.
Do some maths.
1.2 kg of "weight" at 0.4 metres from the pivot is 0.5 Nm of force (these numbers are made up btw)
Clearly you're doing this while standing still, so it won't measure a "rolling" input value. Trying this while riding would be risky and is not recommended.
Example of a spring balance. A digital one probably won't work here.