<p>How long do you normally test an update for Zend Framework before pushing it out into a productions project.  We can break this question up into minor updates 1.6.0 -> 1.6.1 or maybe a major update 1.6.2 -> 1.7.0.  Obviously you don't release it if it add bugs to your code.</p>

<p>Also, as with most other server software updates normally people have a window of time they like to wait and watch the community before even attempting an update on a development environment.  How long do you even wait to start the process?</p>

It seems like the best method would be to have a comprehensive set of tests that exercised all the functionality in your application.  With a good method for testing it seems like you could push it into production pretty quickly.
Another simple thing you can do to help you make your decision would be to simply do a diff against the repository to see what changes where applied to any modules that you use.  If there where no changes, then upgrading shouldn't make any difference.  If something underwent a major re-write, you would probably want to investigate a lot deeper.