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What's a reasonable time frame? That depends on the goal. If the task is one that should be accomplished in days, then a reasonable time frame is a week. If it should be accomplished in a few weeks, then a month is a reasonable time frame. You'll notice that I am allowing a little more time than should be needed. There are two reasons for this.
First, I want to give the employee every opportunity to succeed. I am truly interested in her success. There are times when the likelihood of success is enhanced by taking pressure out of the situation. In this instance, I'm removing time pressure.