This morning I was holding short of the runway awaiting my IFR release when I had to return to the ramp because a passenger's daughter went into labor. I taxied back, shut down, let them deplane, cranked up again, and taxied out on my previously issued IFR (not taxi) clearance. I asked the controller if the previously issued IFR clearance was still valid and he confirmed that it was. At what point would an issued clearance expire or no longer be valid? I'm familiar with a flight plan's time limit in the system but not what happens when a clearance has been issued and you have an unexpected delay.