Classifieds - duration - custom fields

Hi,

I want to offer users only two options for new classifieds registrations

6 months and 1 year.

If I change the custom field entries to these choices ( currently from 1 week to 4 weeks) for duration, will this allow the listings to run correctly for the new durations?

I notice the select box changes to reflect my new options but do not understand how the plugin will know it has been changed and operate with my new options.

Thanks