If you're asking how to setup GA on shopify:
1. create an account,property, view in GA, and Copy the UA code.
2. login to shopify, go to 'online store', then 'preferences'.
3. Paste the UA code into the Google Analytics box in shopify.
If you're asking how to view average time people spend on your pages:
1. Go into GA, and select the appropriate view.
2. Go into the report 'behavior -> site content -> all pages'
3. Average time on each page will be shown there.
Keep in mind that average time on page is not reflective of reality given how GA tracks time on page. Often times, the real values are greater than what GA shows.