Depends entirely on the software, and what's involved in the update. Security updates are prioritized, obviously. Feature updates are considered a lower priority, and are generally installed according to user demand. We're particularly conservative about updating software which may cause compatibility issues for users — for instance, we're incredibly conservative about MySQL updates.
Some software updates are also dependent on the OS. For instance, if a piece of software is distributed as a component of Debian Linux, we will only "backport" a newer version to the current release if there is overwhelming demand for it. (For instance, we backport some tools such as git and svn, but we do not backport core software such as command-line shells.)