After leaving academia, for example at the end of a Ph.D. project, some research and projects might still be unfinished and on-going, and formally leaving academia for a job elsewhere therefore may not be the end of the relation and ties to it.
So, for how long will you continue to work on academic research and in collaboration with your former colleagues? How long is this research-tail after leaving academia?
That obviously depends on several factors that may differ in each case. How much unfinished research and how many on-going projects are you involved in? How willing are you to spend your evenings and weekends on wrapping up the unfinished research? How interested are you in staying in touch with academia and your former colleagues?
On the one hand, if you don’t have unfinished research work, or if you are not interested in working on publishing what remained unfinished when you left academia, the tail could likely be fairly short.
If you do have research to finish and decide to engage in finishing it and getting it published, the tail, on the other hand, could be fairly long.
When I finished my own Ph.D. project and left academia at the end of 2015, I had several research projects that were still on-going. Some were almost done, some were well underway, and others were almost just starting.
I decided to contribute to getting most of them done and published, which extended my research-tail to several years.
In the picture below, I have summarized the number of journal and conference publications that I have authored or co-authored during my years in academia (2012-2015) and during the years after leaving academia (2016-2019). Blue and red bars are numbers related to academia research, while yellow and green ones are related to industry research in the position I joined after leaving academia  .
As can be seen, the blue and red bars extend all the way to 2018, at which point all projects and research from my time in academia had been concluded.
Or in other words, my research-tail after leaving academia was approximately three years .
As mentioned initially, the length of the this tail after leaving academia might differ in each case and obviously could be much shorter.
But if you do decide to engage in getting academic projects and research finished after formally leaving academia, you might need to work on and contribute to this work for several years.
 The number of publications is not, in general, an indication of the quality of the research, nor of the productivity of the individual researcher. Here, I use it as an indicator for involvement in academia research and in industry research, since it is easy to quantify.
 My social-tail after leaving academia is longer, as I still occasionally see some of my former colleagues.