Average Time spent on site OUT OF what...?

This isn't something that is unique to "aver time spent on site", you'll see this for a lot of metrics. The "out of" refers to the entire dimension.

Say for example you're looking at your time spent on site over the course of the last four weeks

Week 1    1.2

Week 2    1.5

Week 3    1.0

Week 4    0.7

The grand total at the top would be 1.1.

Now say that you use a filter on your dimension and filter for "week 1" or "week 2"

Your table would now just have

Week 1    1.2

Week 2    1.5

And at the top you would see

1.35

out of 1.1

The large number at the top is the grand total for the values that are actually visible in the rows. The "out of" refers to the entire dimension that you have in the table.

Yeah, as @mandygeorge said, this is normally the "grand total" for the metric.

Basically, looking at non-averages, you might see:

Which is kind of useless...

Now, if I filter my table:

Then my total will reduce, but still show the original grand total:

Averages, while not a "grand total" will still show the unfiltered average.

You can hide the "out of" per column by deselecting Show Grand Total:

Or, you can shut this off for all columns by default by setting up your preferences... I do this, I really don't need to show the grand total for most uses, and if I do, I can turn it on for the rare times I want it.