Recently in the RT game I just started up the question was asked by my players as to when the imperium asks it's rogue traders to pony up and pay what they owe it. The answer I gave them was every ten years or so they would have to travel back to Terra and pay -10 profit for continued use of their warrant, or -15 if they would be unable to make it to the homeworld and had to meet with a sector's administratum instead.
Think this sounds reasonable?