Since Sufferfest dates all activities with the end time (and is not going to change this) I think you have to finish the workout within the 50h Window. I’m not going to experiment here 
I had a Thread about this: [Feature Request] Set Workout Date to start instead of finish Date/Time
There are countdown timers with the Start and Finish Times here: https://thesufferfest.com/pages/tour-of-sufferlandria-2021-the-stages
The Page also states:
Each stage starts when that date starts on Earth (Which is GMT+14) and the stage finishes when it’s no longer that date anywhere, anymore. Meaning you have a 50 hour window to complete each stage . We encourage you to do the stage within your own time zone, but you can do the stage anytime in between the start and finish times listed below.
Since I’m pretty new to SUF I think I’m going with the Focused (a little bit easier than nuclear) option this year to be safe and will know more for the Tour 2022. I already pimped the prep plan a bit since the current week seems to me a bit too couchlandrian.