as @seanupton mentioned, you must use supervisor AND superlance.
there are 2 main reasons you may want to restart an instance automatically:
you should try to avoid doing it in the first case because, as we discussed on a previous thread, this is a costly process; you better adjust the number of objects in the ZODB cache.
be careful with httpok: I run it only once an hour, because it can restart your instance under some heavy load circumstances and things can get ugly; also, Zope is rock-solid and it's quite difficult to find a blocked instance.
we have instances that have been running for many months, or even years, without restarting them.
here you have a typical buildout configuration for it: