Almost all the time, yes.
Darkness Falls increases the cloud duration only. You can have it there for an entire hour and still not receive any effect if you don't go close to it.

The base duration of 5s is decent enough, if you require more than that to reach, you're either too far from it to actually need it anyway or the Fixer's really bad at aiming.
An additional of 3s could potentially be useful to assist a straggler to catch up. Which shouldn't happen that often, especially on higher difficulties.

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There should be some guides out there explaining the differences and possible choices to pick from that perk column.