I'm getting mixed answers. Will a 250 volts, 140 amperes fuse blow as fast as with 12 volts?
140 amps through a fuse is 140 amps regardless of the supply voltage in a system and fuse will blow because of current through it.
Now the difference between a 12V fuse and 250V fuse is that the 250V fuse can safely handle 250V when it has blown open, but a 12V fuse cannot hande safely 250V because it is not rated for 250V and it may continue to arc over.
At some point, a high enough voltage rating will imply a high voltage drop at rated current, and therefore 12V won't even be able to sink 140A through the thing.
I don't know numbers offhand, but I suspect even distribution line fuses (10kV+) don't drop quite that much, so this will be quite a low order effect, and certainly no 250V fuses I know of, would suffer such a problem.
But that's fusing at all, and you asked about speed.
Notice that opening time depends on fault current, typically well above rated current.
The amount of fault current possible, similarly depends on voltage, and so it could be that a fuse opens faster at 250V than 12V -- because the 250V circuit has a fault capacity of say 2kA and more than enough voltage to force as much through a fault condition; whereas a 12V circuit might only do 1kA into a properly "short" circuit (say a car battery into the proverbial screwdriver), whereas with the resistance of real wiring, an only-mostly-shorted load, and the fuse, maybe it's only 500A instead?
In any case, the correct answer is not some set of assumptions that will inevitably be wrong in enough cases to be meaningless; the point is it can be calculated, given sufficient data. Take the short-circuit fault current, whatever that is -- it depends on source impedance, wiring resistance, fuse resistance, and whatever the expected fault resistance is -- and apply it to the fuse's time-current curve. Whatever the result is, between the two particular applications (say 240V mains vs. whatever the 12V thing is), there lies your answer.
One concern is whether the 250 volts and 12 volts are both AC, or whether the 12 volts could be DC? An AC fuse will (generally) not interrupt DC power. A DC arc through a blown fuse will not extinguish and the power will not be interrupted (until the fuse terminals and holder has been eroded by the arc to a much larger separation than is inside the fuse).
As others have answered, if both circuits are AC or both circuits are DC, the fuse opens when it gets hot, in proportion to:
current * current * fuse_resistance
And doesn't have any dependence upon the system voltage.
Adding to the other answers:
As noted, a fuse blows on the current passed and circuit voltage does not affect the time taken.
But, as Justme (NOT JustMe) notes, voltage rating matters for safety purposes.
In higher voltage circuits where high fault currents are possible, a 10A fuse my blow in well under millisecond if subject to say 100A fault current. However, the fuse needs to be rated to break 100A and to not sustain an arc with the available circuit voltage across the fuse. A 12V fuse may not properly interrupt an arc at 100A in 230 V circuit.
In circuits where very high fault currents are expected and are required to be safely interrupted HRC (High Rupture Capacity) fuses may be specified.
Web search HRC fuse ratings
Typicla fuse time to blow curves for a fast blow fuse family.
I've highlighted the 1 amp fuse curve and added a few intercept points to show the blow times at various currents.
Note that current is the only parameter considered. (In practice ambient temperature and air cooling can have a minor effect.).
Image from here
Fuses are designed to melt at some temperature. This temperature is based on the fuse resistance and amount of current flowing through it. This resistance is selected to the required max. current that the fuse is designed to allow. Voltage rating of a fuse is a safety limit to prevent arc and undesired short circuits).
For your question, a 140A fuse will blow around 140A (A hot summer day will blow fuse just after 140A, a cold winter could delay until a few amps higher). Since current is the result of potential difference (voltage) across some resistance, 250V will blow the same fuse much faster than 12V since it'll reach melting temperature faster.
As an example, see this image showing 25A fuse blown to disconnect a 240V load when drawing power at about 9.2kW, which should have blown at 6kW (if you do the math). Voltage spikes were transient and not persistent long enough to heat up the fuse to melting point.
Notice the gap during which no power was being supplied until new fuse replaced and power reset at under 6.9kW
If the fuse was rated for 12V, the higher 250V voltage may create a short circuit to some other ground, in which case the fuse may stay intact and the short circuit will make fire long before the temperature around the fuse wire reaches the melting temperature for it to blow (from the fire that is, not the current flowing through it).
Some plastics create poisonous gases when melting so in case of fire, so you'll have additional poison and explosion hazard to deal with, in case of fuse material and other plastics or volatile compounds nearby.