Calculating the average time on the site

[mperez5]

Is being a while that I don't do metrics or statistical computations. I have an idea in how to do it but if it is not right am going to make it fool of myself.
How would I get the average time of users on my site?
This is my scenario:

Minutes - Users visited my site
1 - 511
3 - 365
7 - 123
15 - 87
32 - 10
52 - 5
60+ - 68

Thanks any help is greatly appreciated

[unilytics]

You can't get a true average with that breakdown. But here's a way to approximate it:
1 x 511 +
3 x 365 +
7 x 123 +
... (and so on)
That total should be divided by the total number of users (511+365+123...)

The answer I get is ~7.2 minutes per visitor. (error to the plus side)