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The initial appraisal meetings can be accomplished in a couple of hours per employee. In these meetings you'll identify the current level of performance, establish improvement goals, create tracking systems, and define future rewards. If you handle the rest of the process well, your subsequent appraisal meetings can be completed in as little as a 45 minutes to an hour.
The next investment in time comes in the form of establishing feedback mechanisms. In the initial meeting you identified the measures to be used when you established the goals. All that's left is to put a reporting system in place. Keep it simple!