Terraforming takes years, probably decades, from start to finish. I imagine the speed would also vary, depending on how far away the planet was from "Earthlike" when terraforming started, and whether or not the colonists wish to try to keep any native life alive to coexist with the introduced ecosystem. I'd imagine the budget and ideology of the faction doing the terraforming would also make a difference; lore-wise, the Empire is better at genetic engineering and has been quicker at terraforming lots of planets in a hurry, than the slow, methodical, tech-based Federation approach.
There have been several "finishing terraforming" CGs run since the start of the game, but there haven't been any planets that were completely unterraformed at game launch that have since been finished.
I admit to not having closely watched any of the stats of terraforming planets during an update - so I don't know if the devs manually (or automatically) alter the statistics of planets being terraformed to slowly bring them closer to Earthlike stats.