I was being lazy when I just pointed you to the CivStats file. Here is the way to calculate the number of turns (assuming I haven't missed anything).
To calculate the number of turns to complete a job, multiply the "effort" for the job by the basic Movement Cost for the terrain then divide by the number of workers (foreign workers only count as half a worker) and divide by the Work Rate for your government. Also, divide by 2 for Industrious civs, divide by 2 if you have researched Replaceable Parts.
The effort required for each job is:
Mine 12
Irrigate 8
Fortress 16
Road 6
Railroad 12
Plant Forest 18
Clear Forest 10
Clear Jungle 16
Clear Pollution 24
The Work Rate is 1 for Anarchy, 3 for Democracy and 2 for all other governments.
The Movement cost is 3 for Mountains and Jungle, 2 for Hills and Forest and 1 for all other terrains. For this calculation, the effect of roads and railroads is ignored.
For example, if I am an Industrious civ in Ancient Times under Despotism, one of my workers can clear a tile of jungle in:
16 * 3 / (2 * 2) = 12 turns