I have a partial spinal cord injury, the nature of which I mention to just about all my companions. I can get erect without ED meds but pretty often it'll soften up before we are done. I very tiny chip of Viagra makes all the difference, allows for a nice climax & natural fade after. I'm one & done... at least within my time constraints.
A provider once mentioned that V has been scientifically shown to delay climax... study was done with men with Premature Ejaculation where it about doubled the time... from maybe 15 seconds to 30. Not sure it's meaningful to my situation. The lady was concerned that I'd never finish but really it happened right on schedule.
I will often suffer fade if I get distracted my pleasuring her, but TV or outside noises... but recover promptly when the focus is back on me. I suggest a distraction free environment, minimal background music (cover outside sounds with causing me to follow the lyrics).