at my old company I had a co-worker who was moderately competent if he tried but didn't seem to do so all that often.

My boss had been dangling a promotion for me for a few months, and I'd put in some extra work during that time related to my co-worker who seemed to be unable to manage a development team for one of his projects.

promotion time came and even though my manager was very aware that I was doing a significant portion of a co-worker's job, they offered the co-worker the promotion in order to keep them around since they had another job offer.

I think I was gone in about 2 months? didn't take too long to line something better up.