Let's analyze your question. Normally it is forbidden to give a false impression of how much you exerted yourself for someone. For example, the Talmud teaches that if you just happen to run into someone, you may not lead them into believing that you made a special trip to meet them. Even though the person will feel better thinking that you honored him in this way, you shouldn't benefit from undeserved gratitude; rather, you should subtly inform him that he has an exaggerated impression of your efforts.
It follows that if the customer is satisfied to get his repair back after five minutes, but you keep him in the shop fifteen minutes to give the impression that you invested a lot of effort on his job, you are subtly misleading him. Our Sages call this "geneivas daas" — stealing someone's goodwill.
But your case is different. You're not trying to score points with the customer; you're just trying to stave off unfair resentment. In this case the customer doesn't have to know how little time you need to get the job done. (Of course it goes without saying that you can't lie about this.)