Check the links on the Upgrade site for various upgrade methods - Oracle Database Upgrade
The higher the availability requirement, the costlier the solution.
What is/are the source versions ?
Source Version: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0
Target Version:Oracle Database 11g Enterprise Edition Release 188.8.131.52.0
Assuming that the databases are not being moved to a new server, your best option is DBUA. The time required to upgrade the database does not depend on the size of the database - rather it depends on the number of objects in the dictionary. Expect outage for each database upgrade to last about 4 hours. Practice on a server that is identical to production, and clone production databases to the test server to practice the upgrades
MOS Doc 1352987.1