Such language is recommended at the conclusion of all documented
warnings because it provides you with the most discretion on a goforward basis.

Many companies still adhere to the ‘‘calendar’’ approach to
progressive discipline. There’s certainly nothing wrong with this style; however, many employers fear that workers will stay clean
only long enough to get through the probation period—only to
commit the same errors once the probationary window has expired.

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Since this calendar approach is so common, let’s look at how it works. Employers who use disciplinary time windows typically dole out ‘‘stay clean’’ periods in thirty-, sixty-, and ninety-day increments.
Ninety days is normally the maximum for most performance and attendance related problems; most courts would consider ‘‘stay clean’’ periods for longer than that to be onerous.

Thirty days: To closely monitor a poor performer’s work, use a
short window such as thirty days. If the individual is having diffi-
culty performing the essential functions of the job, that should be
enough time to observe results. Shop floor workers, administrative
support staff, and customer service representatives typically fall
under this category.

Taken From : The Hiring and Firing Quention and Answer Book