Dependent on how many categories you have - somewhere around 20-30 links (not your own sites or hosted sites/galleries) per category is a good rough number for content.
With most LL owners PR is not an issue as it really doesnt mean anything anyway.
Traffic will build, so to get listed usually that is not a concern either....just a good clean LL with a good selection of sites for the surfer
and no traffic trading scripts - to start with just good plain hard links