While there’s technically nothing wrong (kind of) with the following screenshot, maybe it’s a good idea to have such periods of time displayed in years instead of days?
Or, better yet, just put something like “too long to estimate accurately” there.
Once we’ve surpassed the expected lifetime of the solar system I doubt the unit matters much 
Don’t be such a pessimist, syncthing is awesome and will most certainly work and be supported still!
However, limiting estimations to time frame of heat death of the Universe might be actually useful. 
Offtopic: I really love the up-to-hour precision on this one:
So what’s the cause here? Is Syncthing totally confused or is there something synthetic/failing to cause these numbers?
No, no, it might be a little inaccurate, but not completely out of order. The story of this particular case is the folder has a mounted FUSE filesystem as one of its subdirectories. FUSE stalled (as they commonly do), so scan speed dropped to zero, and Syncthing keeps increasing estimation (although by this point I suppose a simple “scan speed 0 b/s, has been for an hour, do something” would be a better UX solution).
Yeah. Once the scan rate hits some lower boundary it could better just say Unknown (stalled) or something. And perhaps for long times (while still having a nonzero rate) switch to months and then just > 1 year and leave it at that…
Filed as #5627.