Windjammer is only open for the usual things during the morning and afternoon, and for the buffet dinner from 6pm to 9:30pm. I have seen Windjammer open for the late night pool parties held once or twice per week on some ships, and that was for the theme buffet (Mexican fiesta for example). In that case it was 11pm or so, until 1am. However not all ships use the Windjammer for the pool party and will set up serving tables poolside.
I've been cruising with Royal Caribbean pretty regularly for 20 years and do not recall ever seeing the Windjammer open 24 hours. There was a time they'd have a midnight buffet every night but even then they closed by 2am-ish.