Seems like a simple question, but I've seen it done so many different ways and I'm now in the unique position of having a lot of great data (I think), but can't distill it to a good solid number.

For example, here is a link to the data for users that signed up for our service through a certain channel on 11/1/2011. A lot drop off in the first week, but then the rate of attrition slows.

Given this input data, what's the average user lifespan for these users? How do you compute that?

Thanks for any tips...

Screenshot: http://i.imgur.com/GegBR.png

XLS: http://dl.dropbox.com/u/4278514/user_att_data.xlsx