<p>How often should you use git-gc?  </p>

<p>The <a href="http://www.kernel.org/pub/software/scm/git/docs/git-gc.html" rel="noreferrer">manual page</a> simply says:</p>

<blockquote>
  <p>Users are encouraged to run this task on a regular basis within each repository to maintain good disk space utilization and good operating performance.</p>
</blockquote>

<p>Are there some commands to get some object counts to find out whether it's time to gc?</p>

It depends mostly on how much the repository is used.  With one user checking in once a day and a branch/merge/etc operation once a week you probably don't need to run it more than once a year.
With several dozen developers working on several dozen projects each checking in 2-3 times a day, you might want to run it nightly.
It won't hurt to run it more frequently than needed, though.
What I'd do is run it now, then a week from now take a measurement of disk utilization, run it again, and measure disk utilization again.  If it drops 5% in size, then run it once a week.  If it drops more, then run it more frequently.  If it drops less, then run it less frequently.