A fault on an electrical system will continue until either a protective device operates to isolate the fault, the fault burns itself clear or the power source fails due to the load imposed by the fault, which can occur when the fault is supplied by local generators.
If the circuit has a properly adjusted protective relay and circuit breaker operating are: the relay processing time + breaker opening time + arc extinction. In the case of an unprotected circuit time will be the protection upstream and its circuit breaker. Now if the protection amount not clean the defective this will be maintained if the source is strong enough to break the cable or other element. For instant processing time operations breaker + relay time is something next 50 milliseconds.
In old times when we were using electromechanical relays and old CB's with slow switching auxiliary contacts it was normal for a fault clearing time of more than 3 seconds. Nowadays with the numerical relays and efficient contactors the fault clearing time has considerably reduced and one can go for 40 kA 1 sec also depending upon situations. For ex. if your equipment is feeding a lot of downstream voltage levels like 66kV, 33, 11 and down then it will be prudent to go for a higher time duration in design for the upstream equipment which effectively gives the room for having proper time co-ordination at different levels. Alternately if your equipment is just feeding directly into loads you can design the equipment with a shorter duration fault clearance time.
Fault clearing times can be estimated as follows. First an assumption for the type of fault is made, line to ground, line to line, 3 phase, etc. Additionally the fault impedance is assumed, from a high impedance fault to a bolted fault. Finally a location is assumed from which you can examine the upstream conductoring and distance to the substation, which yields the impedance of the remainder of the fault circuit. With all of these you can now calculate the magnitude of the fault current to expect. This magnitude is overlaid on a protective coordination curve and you can then determine the clearing time by where the fault current magnitude meets the first upstream critical clearing time line.