What do you think, maybe time on site is very important for SEO? For example google probably know how long is somebody on site? And same question for visitors/new visitors...
please be clear with your quetion. what you wanna ask? are you talking about visitor visit lenght? you may check your visitor visit lenght with statcounter it's the best tool
For visitor to stay on your site for long time you should have good content. There is not any fixed period which is important for SEO for visitors to be on your site. Job SEO is to bring visitor to the site. Then you should have good content for the visitor to be on site and to get an inquiry from him or should buy product (for ecommerce site).
Time is not an issue. But you must keep updating your site with fresh content consistently. That's more important.
My english, sorry. Actually yes, this is on-site optimization. But I already get your answer, visitor visit lenght is not important for SEO.