Venus is above the horizon for about the same time as any planet or star... from the time it rises until the time it sets. I think where your flerfer is confused is that Venus one of the few sky objects visible during the daytime if you know where to look - I have observed Venus at near midday on a number of occasions. The star Sirius (Alpha Canis Majoris) is another one that it visible during daylight. However, like Mercury, Venus is inside our orbit, so it never gets more west or east of the sun than the number of degree that equals its maximum elongation. For Venus that's 47° and for Mercury its 18°, i.e.you can only eve see Venus a maximum of 47° "left" or "right" of the Sun - you can see Venus "all day" but not for 24 hours and never more than about three hours after sunset, or three hours before sunrise.
Which is basically the answer I gave to the the flat Earther, but his question related to Venus being visible many hours after sunset. Now it is possible for Venus to be viewed as much as 4 hours after sunset. However the Sun can hover just below the horizon at certain times of year in the north or south. Is it possible therefore for Venus to be seen as a nighttime object for 24 hours, as claimed by this flat Earther?
Well, of course, since Venus is visible during the daytime, if you are standing at one of the poles (the north during Arctic Summer or south during Antarctic summer) then the sun is above the horizon all day every day for a few months. Venus could be visible in the sky if you knew where to look so long as it was far enough away from the sun to escape the glare. Venus has an orbital period of about 225 days, so it would depend on where it is in its orbit during the time of your polar summer. If it was reaching maximum elongation (either east or west) then it should be visible.
However, to see it against a darker sky would be much more difficult. I would have to look up an ephemeris and some orbital data for Venus to be sure, but my best guess is that it is theoretically possible if all the circumstances line up correctly.
1. You would have to be at the pole, observing just before the beginning of summer, or just after the end of summer, when the sun is just below the horizon and there is a long twilight that lasts several weeks.
2. The inclination of Venus' orbit to the plane of the Ecliptic is about 3½°. What that means is that the plane of Venus's orbit differs from the plane of the Earths orbit by 3½° so at its maximum elongation, Venus can appear as much as about seven solar diameters "above" the elevation of the sun (one solar diameter is approx ½°, so 3½° / ½° = 7) .
3.
IF Venus was near its maximum elongation,
AND that maximum elongation also coincided with a point where it was sufficiently separated from the plane of the earth's orbit, then it might be possible for Venus to appear above the horizon in the twilight sky.
These are a couple of rough illustrations to show what I mean by near maximum elongation with maximum or minimum separation from the ecliptic.
Now, have a look at this fantastic photo. This is the December 4, 2021 total solar eclipse taken from Union Glacier Camp, Antarctica, with Venus over on the right.
You can see that Venus is at almost the same elevation at the sun, so this means that at that time, the plane of Venus' orbit was such that it tilted either toward or away from us, giving little separation between the position of Venus and the ecliptic.
