Ad duration is based on category:

Ads appear in the category for the duration specified above (unless they are deleted before that time). An option to repost an Ad will become available a few days before an Ad is set to expire. Reposting an Ad extends its life by another full duration and brings the Ad back to the top of its category. So you could keep an Ad on Gumtree indefinitely, as long as it is reposted before it expires.