The parties’ knowledge, expectations, assumptions and contemplations, in particular as to risk, as at the time of contract, objectively estimated, are among those factors to be considered when appropriateness of application of the doctrine is under scrutiny
In Geipel v Smith (1872), L.R. 7 Q.B. 404 Cockburn, CJ said at p.411:
… the defendants rest their defence on the ground that it was here impossible to expect, from the nature of the circumstances, that the obstacle of the blockade would be removed within a reasonable time. It is a sufficient answer on the defendants’ part that it was not likely to be removed within a reasonable time; and assuming that either party was bound to wait a reasonable time to ascertain whether the obstacle would be removed, in point of fact it was not so removed, and the defendants were therefore justified in not attempting to perform their contract.
And by Blackburn J at p. 413:
… the defendants say 'we are not going to let out ship sail to the port of loading at all, because you, the plaintiffs, never will be ready and willing to perform your part of the contract'. But then it is said, it is possible the blockade might be raised within a reasonable time. No doubt it was possible. But it must be taken on this record that it was not raised within a reasonable time; so if the defendants chose to run the risk, and in the event turn out right, they are in the same position as if they had waited the reasonable time and had then sailed away.
A delay in carrying out a charterparty, caused by something for which neither party was responsible, if so great and long as to make it unreasonable to require the parties to go on with the adventure, entitled either of them, at least while the contract was executory, to consider it at an end.
Universal Cargo Carriers Corpn v Citati  2 All ER 70,  2 QB 401, per Devlin J at p.430:
But a party to a contract may not purchase indefinite delay by paying damages … When the delay becomes so prolonged that the breach assumes a character so grave as to go to the root of the contract, the aggrieved party is entitled to rescind. What is the yardstick by which this length of delay is to be measured? Those considered in the arbitration can now be reduced to two: first, the conception of a reasonable time, and secondly, such delay as would frustrate the charterparty … in my opinion the second has been settled as the correct one by a long line of authorities.