When I am giving a flight review or a check-ride (CAP) I always ask if the pilot calculated take-off distance for today. Sometimes they did, more often I get a response like, "We have 6000 feet of runway, surely that is enough." My response -- and my reason for asking -- is, if you know that on that particular day at that particular loading, that the airplane should reach Vr at 800 feet and you pass the aim point markers or VASI lights but you are not yet seeing Vr, it is probably time to abort the takeoff -- while you still have 4 or 5 thousand feet to stop safely. (Shorter runways have different considerations, but the basic idea remains the same -- do you have enough to take off? To stop? When do you make the decision and what will happen next?) Multi-engine thinking for single engine pilots.
Question for flight reviews: What would cause you to abort the takeoff?