I am afraid that the assumptions made in the article (regarding the distribution) are invalid. One very long (say 2 days) visit will skew the average. And you

I am afraid that the assumptions made in the article (regarding the distribution) are invalid. One very long (say 2 days) visit will skew the average. And you can't infer the standard deviation from the average. For an easy alternative, you could calculate the median time on site ex ante -- let's say it's 1 minute. Then for your ab test, see what percent leave within the first minute. Use that percent and determine if difference is statistically significant.