I am not sure how often the ISS crew gets on ham radio other than to scheduled contacts with schools.
They do also occasionally send Slow Scan TV (SSTV) images from the ISS, but those are also scheduled events.
I do however bounce data signals through the digital repeater (Digipeater) on the ISS.
I can't remember his name, one of the US astronauts that is slated to return to the ISS tries to get on the radio whenever he can, he is a regular guest of Tom W5KUB on his weekly show and at the Dayton Hamvention every year (at least when he is not in space).
I have talked to them several times, a really good time to make contact with them is in June during ARRL Field Day.