In my experience (having both being the recipient of a PIP and leading others through one, too) a PIP should be given a hard deadline of 60 days with weekly milestones. A full 60 days signals the intent of the organization to try to keep a person they want to see on the team. It also provides the employee to understand the kind of shifts and adjustments necessary to work well within the culture of the organization.
Measurable milestones might include:
Daily check-ups with you (that you document in some way) that focus on how they're doing and what you see and what they are feeling/experiencing during this season of change.
Weekly review meetings with your leaders about their experiences with the employee during the past week, especially as it pertains to the PIP.
For the employee – a smile and a positive response to requests. I'm not saying to be sappy and gushy, but to earnestly train themselves to look at each person as someone to serve. Talk about how often you notice or hear about positive changes daily and/or weekly. Encouragement matters.
Ensure projects are early or on time. Meeting deadlines and commitments are important to re-building relational equity. This should be happening even without the PIP.
CC'ing you on all their email communications in response to requests or proactively seeking information about an upcoming project – and reviewing issues/growth weekly.
Review their e-mail/project management communications and help the employee learn how their word choice informs the reader of the tone of the email. Review good and bad examples weekly.
If there is little to no discernible and measurable (even anecdotally) change within the first 30 days, an interim meeting should be scheduled with the employee. Since a PIP is generally the last-ditch effort to keep an employee from being terminated, it is important that they fully understand their progress and not be surprised by any decision come day 60. Using a healthy and clearly defined PIP, the employee's success is truly their choice. That means the employee's change, or lack there of, may very well force the organization's hand to terminate the employment of the employee.
Finally, a PIP should never be punishment. It's important to reinforce this to the employee, who often simply feels misunderstood and isn't aware of how they are interacting (poorly) with other staff. And while it's not about punishment, it is about career and interpersonal development, and helping restore the employee to full trust and relational equity.
Have you used (or had placed upon you) a Personal Improvement Plan? What was your experience? Comment below.