According to this F-16 turn performance chart, at the rated maximum airspeed of 800 knots, an F-16 can turn at about 10.5 degrees per second, so a 90° turn would take about 8.5 seconds. The turn radius would be 9,000 feet. Since you specified a 90° turn to avoid an infinite wall, the pilot would have to start their turn at or before 9,000 feet from the wall in order to avoid a collision.
However, this doesn't tell the whole story. The F-16 can't maintain speed in such a turn, and the reduced speed would in turn reduce the turning radius. If the plane, for whatever reason, absolutely had to maintain its maximum speed, then it would be limited to a turn rate of 3.5 degrees per second and a turning radius of 25,000 feet.