Astral Travel - duration in real time?

[thalcos]

Building a character with an Astral Projection power (from GURPS Psionic Powers), and had a question I couldn't figure out.

Under Astral Travel, it says you can stay on the astral plane for a given duration (i.e. 30 minutes at level 1). But then it says that the duration is always measured in real time (emphasis in the book). I can't quite grok what this means... like is it real time like Luck, that you can use every hour of table time? That doesn't seem right...