Ninety days: To keep employees clean for the longest period of time (for example, with absenteeism and tardiness problems), use a ninety-day window. Remember, you’re notmarried to the employee
for a guaranteed ninety days; if anything goes wrong within those
ninety days, you can automatically move to the next step of discipline (or termination). It’s just easier for employees to be on time for thirty days than it is for ninety days, so why not hold them to a higher standard?

Here’s how to couch these ‘‘calendar’’ warnings to buy yourself the most discretion possible:

Janet, you are now being placed on a final ninety-day warning for
tardiness. If at any time during this ninety-day period you incur two
more incidents of unscheduled tardiness, you may be immediately
dismissed.

By the way, if Janet survives the ninety-day window and then has two consecutive tardies a week later, you’ll still retain the discretion
to terminate her. Most courts and arbitrators would recognize that you’re not obligated to start the entire process over again just because your employee passed some ‘‘magic’’ time limit. However,
if two months go by and then Janet has two consecutive tardies, you
very well might have to issue another final written warning. As with
all cases of progressive discipline and termination, individual cases
must be determined on their own merits. When in doubt, confer with appropriate legal counsel to review the specifics of your case.

Taken From : The Hiring and Firing Quention and Answer Book