Logically it would be running at all times but like a planes black box it would be impractical to hold an infinite amount of data about the cars operations (where would it all go). It would maybe hold the last 15 minutes of data (just a guess but u get the gist of what i'm saying) and keep on writing over the data as it goes. Or it may even just hold maximum values. ie, highest rev u've got it to, highest speed etc.