The comment 5 in https://bugzilla.redhat.com/show_bug.cgi?id=990990 tells that the JBPM cluster checks his nodes every 20 seconds by default. This information + how to configure that period + some best practices should be available for the user in the documentation.
@Radek: the fixed version is here (see the admonition at the very end of the section): http://docbuilder.usersys.redhat.com/14147/#Clustering_backend_database thanks for all your help :)
Verified (ER4)