How to change/edit 'Duration' fields in Classifieds plugin?

Hi There, in Classifieds plugin, how can I change 'Duration' filed not to show 1-4 weeks. I would like for Classifieds ads stay till removed. Is there a actual scrip that will expire ad after 4 weeks? Can I disable this option? Or even better can I use this filed for something else I would like to display on the ad? For example some Classifieds category?
Thanks, Dean