Check out the code in this thread:
https://www.b4x.com/android/forum/threads/see-human-readable-arduino-running-time.107735/
According to this with the maximum number of Ulong as well as the millis as ULong you get:
1193:02:47.295
Which corresponds to:
49 days 17 hours 2 minutes 47 seconds and 295 milliseconds.
What happens in the next millisecond? Does it go back to zero? I suppose the register does not have enough positions and a bit is lost in the beggining living all the zeros... Right?