How often does the average visitor log in, I wonder?
Difficult to tell.
The site gets around 135,000 visits per month. In the past month there were around 1,600 active members. If we assume that each time a member logs in it is counted as a new visit and then we assume that the ratio of members to guests is 40:60, then 54,000 of those visits were from members.
That would make it nearly 34 visits per member per month ... or an average of just over once per day per member.
This is only a very rough estimate ... the ratio of members to guests is a guess based on the number of visitors who access the site via a search engine vs visiting directly ... which is not always accurate - many members type "Somersoft" into Google to get to the site !!!
But anyway - I think it sounds about right from a ballpark measure ... the members who logged in over the past month did so on average slightly more than once per day.
I would have to do some programming to capture more accurate statistical information about login frequency and such - that would be an interesting exercise though.