Whilst heathfield has made an attempt, figures would be largely ubstantiated since no records would be kept as most detahs would have been deemed inconsequential.
However, just a note on the figure provided by heathfield. There have been four major walls:
208 BC (the Qin Dynasty)
1st century BC (the Han Dynasty)
1138 - 1198 (the Five Dynasties and Ten Kingdoms Period)
1368-1620 (from Hongwu Emperor until Wanli Emperor of the Ming Dynasty)
The 1m figure refers to the building of the 1st wall in the Qin dynasty where 3m people (about 70% of the population) were involved in its building. The wall was extended by 1500 miles after this.