Not very. There are a lot of variables that determine the caloric expenditure of an individual, and a piece of cardio equipment is not usually going to get it right.
Many will usually ask for your wt/ht/age to help determine, but keep in mind, the number is still just an estimate.
Rather than focus on caloric expenditure of your cardio workout, focus on duration and intensity (how long and how hard you work).
Think about how how long you worked out (duration), and your heartrate/sweating/ breathing heavier than normal - (can indicate intensity).
That is a better indication of the quality of your cardio session.
