Q: "What is considered average/good/exceptional for Pages per Session and Average Session Duration on my website or blog?"
A: Pages per Session and Average Session Duration metrics depend on the site and its purpose. A better question to ask would be: "What is my rate and trend? And is that a good thing or bad thing based on my site goals and objectives?” For example, a publisher’s goal would be to have many page views and longer session duration so they can generate ad revenue. Whereas an e-commerce site would most likely prefer to have visitors come to the site, add to a cart and checkout with no distractions.
For blogs, since they typically are purely informational, one can assume the user arrives to the site, absorbs content and leaves. This is why blogs tend to have a high bounce rate, which in this case, may not be a bad thing. But it ultimately depends on the goals of the blog. If you're looking to increase pages per session and average session duration, consider having repeated calls to action on the blog to urge the user to explore more content or subscribe, learn more, etc.
Ultimately, in order to determine if your website is successful, you should have defined goals in place. Then you can develop a deeper understanding of the effects of tactical decisions and how they play against the strategic goals.