When using Alfresco with a LDAP Authentication Subsystem, currently the default Java timeout is used for LDAP queries, which is quite long. Java does support specifying a timeout for LDAP connections / queries, see http://docs.oracle.com/javase/tutorial/jndi/newstuff/readtimeout.html for details.
If you wanted to set a timeout, it is currently rather fiddly to do. Unlike the other configuration values which can just be set in your properties file, you would have to override the whole of the ldapInitialDirContextFactory bean definition, to be able to list the additional property map entry.
It would be much better if an additional property were added for the timeout, initially set to 0 (the current Java default). This could then be quickly changed if needed in the properties file for the authentication subsystem, rather than having to copy and paste large amounts of spring config to set one more property.