Really it all depends on the rocket and guidance system used. A typical gravity turn starts within seconds of launch - basically as soon as the rocket is clear of any ground support equipment - to increase stability(lower speed means less drag to counteract and velocity vector changes more quickly) during the "active" portion of the gravity turn(where the rocket uses thrust to turn itself). It doesn't take much turn either (pitch is usually 85-89° at the start) before gravity takes over and the rocket can follow prograde the rest of the way up while still turning(and not using thrust to do so).
Basically, any rocket using a gravity turn will fly along a high angle ballistic trajectory through the atmosphere, starting near vertical and ending near horizontal by the time it leaves the atmosphere(60km in game. About 70-100km irl). This is the most efficient approach to orbit because you gain the most altitude as early as possible to decrease drag, but then begin to arc over to a relative horizontal plane to reach orbital velocity asap, all the while not using a drop of fuel beyond the initial couple degrees of turn to accomplish it.
The exact altitude to start would basically be the height of the rocket or launch structures or some small multiple of that height. Alternatively, you could use a velocity as your turn point to ensure that the rocket is moving fast enough to complete the maneuver safely as opposed to waiting for it to reach a certain height even if its at 1m/s. The end of the gravity turn depends on the orbit you want to reach. Basically it should be the final periapsis you want for your orbit, or any value larger than 60km to ensure that you've left the atmosphere by the time you're horizontal
Really it all depends on the rocket and guidance system used. A typical gravity turn starts within seconds of launch - basically as soon as the rocket is clear of any ground support equipment - to increase stability(lower speed means less drag to counteract and velocity vector changes more quickly) during the "active" portion of the gravity turn(where the rocket uses thrust to turn itself). It doesn't take much turn either (pitch is usually 85-89° at the start) before gravity takes over and the rocket can follow prograde the rest of the way up while still turning(and not using thrust to do so).
Basically, any rocket using a gravity turn will fly along a high angle ballistic trajectory through the atmosphere, starting near vertical and ending near horizontal by the time it leaves the atmosphere(60km in game. About 70-100km irl). This is the most efficient approach to orbit because you gain the most altitude as early as possible to decrease drag, but then begin to arc over to a relative horizontal plane to reach orbital velocity asap, all the while not using a drop of fuel beyond the initial couple degrees of turn to accomplish it.
The exact altitude to start would basically be the height of the rocket or launch structures or some small multiple of that height. Alternatively, you could use a velocity as your turn point to ensure that the rocket is moving fast enough to complete the maneuver safely as opposed to waiting for it to reach a certain height even if its at 1m/s. The end of the gravity turn depends on the orbit you want to reach. Basically it should be the final periapsis you want for your orbit, or any value larger than 60km to ensure that you've left the atmosphere by the time you're horizontal