A time period you specify applies the rule to all time periods with the specified duration. The duration is a number of days. In other words, the rule must be true for any time period specified. For example, if you create a rule that says a maximum of 3 offers can be sent to any customer in a 30-day period,
Optimize does not allow any optimized offers (also considering offers already given in contact history) to exceed 3 in any 30-day window. This "sliding window" interpretation of the time period allows you to truly control your offer presentation strategy.