It's only in-progress transactions that will block the DDL. Any new transactions you start after issuing "alter table ..." will work as normal and allow the DDL to complete.
So if you're in a "true" OLTP system, you should be able to keep the timeout reasonably low.
If you're wanting zero downtime upgrades I strongly recommend you check out Edition-Based Redefinition (EBR):
https://docs.oracle.com/cd/E11882_01/appdev.112/e41502/adfns_editions.htm#ADFNS020 https://oracle-base.com/articles/11g/edition-based-redefinition-11gr2 http://www.oracle.com/technetwork/database/features/availability/edition-based-redefinition-1-133045.pdf