Site Owners - How Often Do You Test Things?

I was wondering how often you paysite owners testing changing things on your tours and join pages, prices, text, images, etc, etc?

Right now im putting together a couple of new movie sites and im wondering just how many times i should change out the 'episode' boxes on the tours?

Do you think it is worth changing them out every couple of weeks, days, months, etc?

Right now i think im going to use a couple of php includes to automatically change the preview images every couple of weeks and gauge how well each of these changes work in terms of getting sales based on the pics with the ultimate goal of having no further changes once i find the image sets that pull the most sales from the tour.

What things have you folks played with on your sites and how did you do it, automatically or manually?

Regards,

Lee