The NATS bond is 2 fold.
On completion of your basic training (when you get your first student rating) at the college you are bonded for 3 years, the bond drops by £1000 per month, so is a maximum of £36k if you quit directly after your student course.
On validation the second part kicks in which is the same thing again, drops by £1k per month for 3 years, so three years after validation your bond is gone.
If your training is terminated at any point the bond does not apply. Also all the above is from memory so there may be errors!