There’s an open feature request specifically for time tracking: https://www.drupal.org/project/farm/issues/2402943, and a branch has been started for exploring this as a new “quantity type” (for farmOS 2.x): https://www.drupal.org/project/farm/issues/3196844
In the meantime, the best way is probably to just create a log that represents the task, and one or more quantity measurements on the log with a “Measure” of “Time” to represent the time spent. This will at least allow you to get the data in, but it isn’t great at separating out WHO did it, if you have more than one person working on task. That’s what the “Time Quantity” will add: a user reference field on the quantities themselves.
This forum topic has some other related discussion: Additional log "time" features - any interest?