Probably less than 6 years. I'm not sure exactly when it was kicked into the long grass but probably around the time of the Hendy review which I think was early 2016. The original announcement that it was happpening at all was December 2013.
I think it would have been when all electrification schemes were paused in summer 2017 when GWEP (in money terms) blew up. So a ~4 year "pause". Although no formal statement was ever made specifically mentioning Bolton-Wigan, it was just "implied" paused at the same time.