Google Analytics calculates average session duration by dividing the total duration of all sessions (in seconds) during a specified time frame by the total number of sessions during that same time frame.
A good average session duration, then, might be anything above three minutes!
This is a SIMPLE and CHEAP way for increase it!
Don't worry for the bounce-rate! The Bots will perform actions on your site: by visiting other pages of your website, clicking etc.
So your bounce-rate will decrease.
You can't use these services:
1) Altervista
2) Blogger
3) Wordpress (if you have domain.wordpress.com)
You can send traffic on:
Your website or a page of it