probably about every six months. I am sure it varies. I had not used my discount tire card since 2011 and made my last payment in jan 2012 and its still open. I bought a tank of gas at exxon and it still worked.
An alternative is to set some recurring bill like water, electric, phone, cable, etc. to your cards. That's how I keep 6 of mine open without having to ever actually use them.
I haven't used my Macy's Store Card in two years and it's still open.
I just got approved last week for a Best Buy store card with a $2500 limit. I charged $2200 worth of electronics on it for the new home I just purchased. I plan to PIF within the next 3 weeks. I don't go to best buy all that often but I would like to keep the card open to build some history. I was wondering how often I need to make a small purchase so they don't close it? Thanks. :-)
I 'd do it at least every 4 months. I know they account review every 6 months and keeping moderate activity on it should keep you in line for auto-CLI's.