How long do you wait before seeing rank improvement after pbn links ?

Have you actually checked that the homepages of these 8 domains have been recached in the last 2 weeks? When adding a new piece of content to a PBN site it will usually get crawled and indexed fairly quickly given a decent enough root domain authority but this does not mean that the homepage is recached every time a Google spider visits.

Check the cache of your 8 homepages and let us know if the new links you've added are visible. If they are not then voila, you have your answer.