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When you cannot estimate the time it would take to finish a job because you have no clue of the complexity of the problem, or the solution is dependant upon various intermediary results, then it pays to put a time limit on the amount of effort being invested in solving the problem.
When you cannot reasonably estimate the time it would take to finish a job.
Identify the first step towards the solution to the problem. Put a fixed amount of time to be invested for the first step. At the end of that time frame, revisit the problem with the experience you have gained and decide the next step.
The fixed amount of time should not be too large. Try not to have it longer than two weeks.
This kind of approach helps you to get started with tasks whose solutions are not known upfront. It prevents us from shooting in the dark for large periods of time. It is one of the approaches to fail-fast.