XCSoar knows about the glider's ballast and the dump time. It would be a useful
addition to XCSoar if, on the final glide, it warned a pilot to start dumping
ballast in time to empty the glider before completing the task.

The calculation should probably use the current average speed over the last
minute and then weighted up or down to allow for the height excess/difficiency
over the desired arrival height. I would also add some sort of safety margin to
this time (maybe 30sec).

I would see the warning being issued about 1 minute before the dump would
commence and then rewarn 10 secs before the dump is to start.