You only need to restart when there is a kernel update, and the frequency of kernel updates depend heavily on the distro used. Debian stable, for example, although using ancient versions of the packages, is a great OS for such a use case, as kernel upgrades are really infrequent. Have a look at the changelog frequency of Squeeze [1] or Wheezy [2].
[1] http://metadata.ftp-master.debian.org/changelogs/main/l/linu... [2] http://metadata.ftp-master.debian.org/changelogs/main/l/linu...