Right ascension (RA) is measured in hours, minutes, and seconds, and it goes from 0 hours to 24 hours. Therefore, RA 18 hours occurs at 18 hours on the celestial sphere.

However, you might be asking a different question. Here are some possibilities:

* What time of year is RA 18 hours at the meridian? Since the Earth rotates, different parts of the sky are visible at different times. If you're asking about the time of year when a celestial object with RA 18 hours crosses the meridian (highest point in the sky), that would be around September 23rd. This is because the Sun has RA 18 hours on the autumnal equinox.

* What time of day is RA 18 hours at a specific location? The exact time when a celestial object with RA 18 hours is visible depends on the location and the time of year. You'd need to use a star chart or astronomy software to find the precise time based on your location and the date.

Please clarify your question if you're looking for something different.