The way feeds work, we don't seek out all the posts by groups/users you follow every time you load the page. That would be slow, instead when users/groups post, we find everybody who follows them, and drop that post into their own little feed bucket.
The moment you leave a group (give or take a few seconds), we stop sending things from that group into your feed. We don't, however, remove existing posts from your feed.
I'm not certain, but I believe that if somebody bumps the post, it will get bumped on your feed, as long as it remains in your feed bucket, which gets trimmed regularly to a certain length. Again, I'm not 100% certain on this behavior.
Anyways, TL;DR you will stop getting new posts from the group immediately when you leave the group, but existing feed posts will disappear over time as new feed posts from other users (which you still follow) appear in your feed.