It really depends. Ottawa's 8-km line took about three years. Here's the timeline:
Timeline
August 1998. Council of the former regional government directed staff to proceed with the development and implementation of a pilot project for light rail. A year-long study of light rail including an environmental assessment was conducted.
September 1999. City council approved the light rail pilot project and an agreement was negotiated with Canadian Pacific Railway (CPR)
2000. Light rail project (Capital Railway) incorporated as a federal railway
2000-2001. Lines and signalling system upgraded and installed plus new stations constructed
January 2001. City took delivery of the trains
April 2001. City began using the CPR lines for testing and training
October 15, 2001. O-Train began operational service
from:
http://www.tc.gc.ca/programs/Environ...ailproject.htm