When you set up your Leads profile you have the option to decide how long the search data will remain after a search in a campaign. You can set under the Total Antiquity (Months), and choose 1, 2 or 3 months to retain your search results.
The longest you can keep your search results is 3 months.
The system automatically defaults to 2 months. After that time elapses, the search data will be deleted.
Once the system purges data it is gone forever, and you would have to re-run your search to obtain results again.
A good workaround would be to export your search data into a CSV file that you can save.