I found the solution.
We had to apply an weblogic patch (from weblogic 12.1.1 to weblogic 12.1.1.0.5).
In 12.1.1.0.5, i could access the RMI_TIMEOUT property. Thats why it was deprecated in 12.1.2, even if it does not exists in 12.1.1 raw version. It becomes available after the patch is applied.
# EDITEDIf i use the weblogic proprietary API, the rmi timeout works in any version:
Environment env = new Environment();
env.setProviderUrl("");
env.setInitialContextFactory("weblogic.jndi.WLInitialContextFactory");
env.setRMIClientTimeout(30000L);
Context context = env.getInitialContext();
Hope it helps someone. Tiago