Negative Times

What is the prescribed period of time to descend through a "negative" movement?

Usually 10 seconds. Sometimes it isn't specified: As far as I recall, HBP16 is just a negative, without any time requirements.

10 seconds as ravn said. it's also important to maintain a fluid and constant tempo during all the descent, weak points need to be reinforced.

I just base it on the course video if not specified.

 

For example in FL PE15 the negative shown is only ~3 seconds not 10.

 

Obviously, the slower and the more control, the greater the difficulty and benefits from the exercise.