How long is a solar day on Mercury?

176 Earth days

https://science.nasa.gov/mercury/facts/

Mercury's solar day is 176 Earth days; this is because Mercury rotates around its own axis much slower than Earth does. Thus, although Mercury's year, which is one turn around the sun, is only 88 Earth days, its solar day is over twice as long. In that way, you would have to spend over two of Mercury's years on that planet in order to have been there a day.

https://coolcosmos.ipac.caltech.edu/ask/23-How-long-is-a-day-on-Mercury-