It depends on many factors (number of nodes, size of the application/database, number of modules, number of workers, ..etc). The best practice is to try this on a test instance, estimate the time it would take before applying this on your production instance.
Btw, R12 (12.1.2) is available which contains many fixes, I would strongly recommend upgrade to 12.1.1/12.1.2 instead of 12.0.6
Oracle Applications Upgrade Guide: Release 11i to Release 12.1.1
http://download.oracle.com/docs/cd/B53825_03/current/acrobat/121upgrade.pdf Note: 761570.1 - Database Preparation Guidelines for an E-Business Suite Release 12.1.1 Upgrade
E-Business Suite Release 12.1.2 Now Available