The last repetition in the set should be the last one you can do with proper form. So if you could keep going, the weight is too light. If you can't get close to the end of the set before you are sacrificing form, the weight is too heavy. You want it to be challenging, but not so much of a challenge that you risk injury.
Do you measure your heart rate during your elliptical workout? That would be one way to know whether or not you're working hard enough. Here's an article on exercise intensity you might find helpful: www.sparkpeople.com/resource/fitness_artic
Hope that helps,
"You may have a fresh start any moment you choose, for this thing that we call "failure" is not the falling down but the staying down." Mary Pickford
"No matter how slow you go, you are still lapping everybody on the couch."