When they go to full impulse. Kirk does a captain's log where he says this.
This got talked out quite a bit 30 odd years back, and again in the last 10 or 12 after the dvd.
Consensus was this reference really meant 'risk engaging UNTESTED warp engines while still in-system.' Presumably you'd get towed out of the system and test calibrate engines out there under normal conditions.
In system warp is just always a given in TREK, though Moore obviously was referencing Diane Duane (again!) with the going to warp too close to sun bit on DS9, this time tapping her THE WOUNDED SKY instead of MY ENEMY MY ALLY, which has an awful lot of points of similarity to his Romulan defector script.