I ask because I assume that most link directories are pretty pointless to submit sites to unless your site is fairly aged.
Yes, as soon as you deem your site completed enough. Also, please take note, that most dir take time to accept your submissions, and then the dir page on which your link is must be indexed before it start counting for your site.
I would start submissions as soon as possible, do things which do not rely on manual review first and as you work on your site at the same time and it gets better start the sites who may have a manual review process. I have found you have a lot better chance with directories with manually submission if the site is actually good.