If it doesn’t work for you then you may need to change some PHP settings at the server, or get your web host to do it. Its not something kiwitrees can control.
The code to clean old sessions is executed when a new page is fetched. It does not happen every time. Perhaps 1 in 100 or 1 in 1000. This is controlled by the “gc” (garbage collection) settings in your PHP.INI file “gc_probability” and “gc_divisor”.
If your site has very few visitors, you can change this to a lower value.