Google Analytics measures visit length by tracking the amount of time a visitor spends on one page on your website to another page on your website. If a visitor only visits one page, then Google Analytics (and Leadfeeder) will display that visit information as a length of 0 seconds and a 1 page visit.
While the visit was clearly more than 0 seconds in reality if there isn't another page visit later to indicate a 'stop' in viewing the first page no time on the page can be established.
A good way to visualize this is as a horse race. The starting point is the starting gate. The end point is the finish line. You can easily measure how fast the horse got from the starting gate to the finish line because they are both available points. However, if the horse was never seen at the finish line it is possible it never started running, or perhaps never stopped at the finish line. Those two points are vital for measurement.
When measuring visit length on a website you can consider the landing page the 'starting gate' and a new page visit the 'finish' line. Without those two points the time can't be measured.