Its when any relationship (plutonic or not) runs its course and my morals and ethics tell me its no longer of any value to me.
I cant speak for other Leos but for me once this happens there is no turning back. I go through a phase of self reflection and look at where I did things wrongs and tend to blame myself.
I find it hard to let go but once I have in my mind I have logically pieced the puzzle together and then realise there is no value left in that relationship for me. Its at this point I stop thinking with my ego and pride and carry on with my life.
The thing with Leos is that we placed a lot of value on morals and ethics and if we have acted morally or ethically then the other person is done - out of sight out of mind!!!