Either change the global container configuration for stateless session beans in conf/standardjboss.xml or define your on container configuration in the local jboss.xml for your EJB.

The two parameters of importance to you are remover-period and max-bean-life.

The first indicates the period of the timer task that will scan the cache and
the passivation storage (currently the filesystem) of stateful beans in
search of beans that have age > max-bean-life. If it finds them, it removes
them, from the cache if they're there, from the storage if they have been
passivated.

<cache-policy-conf>
 <min-capacity>50</min-capacity>
 <max-capacity>1000000</max-capacity>
 <remover-period>1800</remover-period>
 <max-bean-life>1800</max-bean-life>
 <overager-period>300</overager-period>
 <max-bean-age>600</max-bean-age>
 <resizer-period>400</resizer-period>
 <max-cache-miss-period>60</max-cache-miss-period>
 <min-cache-miss-period>1</min-cache-miss-period>
 <cache-load-factor>0.75</cache-load-factor>
 </cache-policy-conf>