Paradox is the literary device used in the sentence above.
Explanation:
The term "paradox" means "contrary to expectations, existing belief". It is a statement which appears to be a contradiction, but when investigated may be proved to be true. In this case the contradiction has to do with the fact that the speaker asks the addressee not be too long but, at the same time, he/she promises to wait for him/her all his/her life. Waiting for someone all your life will be long for sure. That's the contradiction.
