I always give a heads-up about how long I can last. The reaction is usually the same—first the scoff: “I got this, I know what I’m doing.” It doesn’t usually kill the mood. Right before things get intimate I kiss their neck, gently nibble an earlobe, and softly whisper not to finish before I do.
One time the other person finished early and assumed we were done. I kept going until I was finished too, which took longer than they wanted. They seemed annoyed when they left. I had warned them not to finish first. They got theirs—now it was my turn.