If you've configured it to generate an interrupt on completion, it will do so at the end. But you won't have an end unless there's a beginning. YOU have to select the input you want to measure, and then start the conversion process. Then go off and do something else. When it's done, it'l let you know.
But the interrupt wont happen unless you start the conversion process.
If the conversion starts without anything connected to the ADC input, the conversion will still run, and cause and interrupt at the end, but the results will be meaningless.