So while this sounds like a classic market sizing, knowing a bit about the reality, they clearly fail in this case. There is no one wall, but many different parts, partly connected, partly not, made with different technologies, mud walls, wodden pallisades, bricks, etc...with hundreds of years in between construction, supervised by different cultures, using different technologies.
But in order to be helpful here, you'd want to estimte length, widths, height, maybe split the lengths into parts that are built on different terrain (e.g. 300 km from Tianjin to the border of inner mongolia with 50% hilly terrain, x km through dessert & grassland, etc. and assume different building speeds. That would get you to a number of person-years of building time.
Then you can assume number of available workers, estimate and come up wiht a rough estimation of how long it took them to build the wall.