Over a month, when Jamil attempted to clone DFH 120 on the "can you brew it?" podcast. That beer requires a ton of daily supplemental sugar additions in the primary. That doesn't include the diacetyl rest, just pure primary fermentation.
I had a lager that I fermented at 48F. It took 5 days for active fermentation to start, it took a full 14 days to get to 75% of FG, two more days of active fermentation in the D-Rest to get to FG. 15 full days of visible airlock activity.