how often should the boxes be rebooted

As I have mentioned before, I don't actually use the primary box for TV viewing in my whole home DVR setup, but do everything through the 2 auxillary boxes so that I can record 2 shows at a time as needed. So I can never tell when the main box is having a problem until it fails to do something (like not recording shows and giving the explanation "Couldn't find in the guide"). So I am wondering, since rebooting the boxes seems to be the solution most times, how often should this be done. At this point, I and my wife have decided to try every monday morning before going to work depowering everything and then when get home, repowering up to see if that stops any of these missed recording problems.