Which literary device is used in the following sentence? "If you are not too long, I will wait here for you all my life."

The correct answer is paradox.

Paradox is a figure of speech that combines two completely opposite things into one. In the excerpt above, we can see that the speaker is willing to wait for his/her loved one his/her entire life, however, only if they don't take too long to get to him/her. This is a paradox because they weren't willing to wait for them long in the first place, as it turns out.

This is an example of a paradox.

He/she would wait for his/her's person of interest their entire life, of course, if it's not that long. It is a clever paradox.