I doubt it measurable in time or in distance beacuse almost each corner is different with at least different radius.
They descibe it perfectly in twist of the wrist vol 2 dvd.
Twist of the Wrist II DVD | Keith Code, California Superbike School (amazon has it aswell)
They call it two step technique
1. estimate a turning point on the road (temp. reference if you don't know the road) - where the corner begins
2. when closing on to that turning point find mid-corner position where you want to clip it on the inside (Apex)
They are mentioning it that you should be looking at that point WITHOUT actually turning your bike in, stay wide do not cut the corner, your're pointing your head-eyes torwards that point.
In the longer corners since you don't see correct apex at the corner entry it's temp. aswell. You're constantly looking to inside of corner searching for the correct Apex. After you establish the Apex you lean on and clip the Apex.
So if you mean looking into the turn as to when you turn your head torwards the inside i would do it before my "first step" remmember that at that point your're looking into the corner not turrning your bike in that same moment, head and eyes lead and the bike follows. Remmember that you do not want to leave your eyes on that specific point, you're establishing it, pointing bike to clip the Apex but you're at that time looking futher to the corner/road, do not lock your eyes on one point!