How long should I hold stretches generally

Simone T.
It depends for the reason why you do it.
For example: you can stretch in the morning to loosen up your body after being still while sleeping and for this reason you are ok with short active stretches or
do it in the evening to relax your body and preparing to go to sleep with some passive stretches or
you can do a mix of the two during the day as a workout with the goal of increasing your flexibility and in this case you do multiple stretches for a longer time.

Chip N.
About 9-18seconds, because it makes you relax and the body gets at least a little time being in a position that it usually is not, and because your body is getting rid of tension, stretching out, etc.