The Porsche tiptronic is am adapted MB NAG 1 722.6. Until 2005 Mercedes Benz recommended to do ATF oil and filter changes first at 90 K miles. And than no more. They've called it a lifetime fill.
In 2006 they had enough experience with bad transmissions of MB cab drivers. They've changed the ATF and ATF oil filter interval to 60.000 Km. And no longer a lifetime filling.
60.000 Km is 37.5 K miles.
Hope that helps.