Ad- hoc evaluation is a quick assessment method to evaluate a team, person or a job title by choosing evaluator(s) for a particular period defined by you.
Periodic evaluation is an assessment method to periodically evaluate people on selected result areas. This period can be defined using daily, monthly, weekly, and yearly divisions. The duration is defined by providing a number to indicate the duration between the divisions. The defined period is taken as the duration that you want to evaluate.
For example, you could take a monthly division for a duration of 3 months, or you could take a daily division and have it sent out every 10 days. This should provide you the flexibility to evaluate periodically for whatever period you deem fit.