Updating from Joomla 3.4.5 to 3.4.6 is a simple one-click update. Unless you have core hacks (not recommended), the update process is absolutely safe. But please update, because this fixes a critical security issue.
Here's from the Joomla Release News:
What's in 3.4.6
Version 3.4.6 is released to address four reported security
vulnerabilities and includes security hardening of the user password
reset system. Security Issues Fixed
High Priority - Core - Remote Code Execution (affecting Joomla 1.5 through 3.4.5)
Low Priority - Core - CRSF Hardening (affecting Joomla 3.2.0 through 3.4.5)
Low Priority - Core - Directory Traversal (affecting Joomla 3.2.0 through 3.4.5)
Low Priority - Core - Directory Traversal (affecting Joomla 3.4.0 through 3.4.5)
As for the time frame, I estimate 4-5 seconds (once you backed up your site...)