That definitely depends on where and which kind of data center. If you're willing to take a derelict warehouse and put in containers like Google did, all you need to do is providing power and fiber (which should be plenty in any industrial settings) and you're set in a matter of weeks to months.
If you're aiming higher, as in design a DC, buy the ground, build the building itself and then installing all the stuff needed, you're in for much more money and time. Depending on the local politics and laws as well as power/fiber infrastructure, you can cut some corners but the worst-case (uncooperating politicians that need to be brought in line by the courts, the next 110kV transformer being at capacity, no fiber and no empty tubes in the ground which means digging yourself) is the benchmark there.