Normally debates about the job-creating punch of a given policy initiative are incredibly controversial and hard to adjudicate, but in the case of the Keystone XL pipeline the issue actually seems pretty clear. It would require about 13,000 person-years of work to build the pipeline, probably over the course of about two years, plus maybe about 7,000 more person-years manufacturing the stuff the pipeline is built out of. It's possible to spin this out into a great big controversy, since the pipeline's proponents have decided to spin that as "20,000 jobs" when it's almost inconceivable that 20,000 discrete individuals would be employed on the project.
In their more ambitious moments, however, pipeline boosters have tried to claim that hundreds of thousands of long-term jobs will miraculously appear like manna from the heavens due to lower gasoline prices, which is nuts. On the flipside, the criticism that these Keystone construction jobs will only be "temporary" is a bogus complaint—that's the nature of construction jobs. You have a job, the job is done, then if you're lucky you get another job.