How do I create an expression that only lasts a certain amount of frames?

Alternatively, you can use Expression Timeline. It can combine as many expressions as you want and even create smooth transitions from one expression to another one or from an expression to keyframes.

Here is a tutorial showing how it works (actually combining several expressions from Dan):

Limit Expressions to a Range of Keyframes with Expression Timeline | mamoworld

It'll be something like this:

tStart = 11;

tStop = 17;

if ((time > tStart) && (time < tStop)){

( your original expression goes here)

}else

  value

Set the values of tStart and tStop to the appropriate times.

Dan