Of course it's all individual - your fitness, exercise, diet etc etc will all affect your ability to cum on a given day as well as your sexual habits.
Re habits - for a youngster who could literally jizz in his pants with the slighest provocation
abstinence will make things much worse - probably better to whack off before going on a punt! For older guys you might benefit slightly from abstaining for a few days. But my experience as someone in mid-40s is exactly as you suggest US - any more than 36-48 hours makes zero difference to my ability to cum.
That said, if you get stuck in a rut of always cumming the same way - especially watching porn and with a death grip wanking style - then you will train yourself to only cum that one way and may struggle.
Ultimately variety is the spice of life. When I'm regularly fucking - ie at least two or three times a week - I'd say that if anything, having cum once, my chances of cumming again are definitely improved by cumming more often whether through wanking or regular sex - my body just gets used to recovering more quickly. The key is not to get stuck in a rut of always cumming the same way - whether masturbation or with a partner.