The jboss client UserTransaction implements Unreferenced
so it will be recovered when distributed garbage collection
After your reply I did some further tests. And basically yes - I do get a Rollback about 10-14 minutes later, and an error is logged "[UserTransactionSessionImpl] Lost connection to UserTransaction clients: Rolling back 1 active transaction(s)". This is very good and answers my concerns.
I assume that the java.rmi.dgc.leaseValue is important - it is set by default to 10 minutes - and the rest of the delay depends on when the gc kicks in.
I have no idea how I would change the RMI leaseValue in Jboss, but am not very worried about that - 10 minutes seem fine to me.