I like to (jokingly) say that it's a fair 50% :
0% when used by player
100% when used by AI
Wich averages at 50% !
No, really, we just don't pay much attention to it most of the times, we only notice when it screws us.
If you are already winning, chance being on your side doesn't affect the outcome, so you just tend to forget when it proced for you.
If you are loosing a bit and it's saving your game, you will often feel you could have won without it, because we are used to win way over 80% games.
If it really saves your ass, you will notice, but that's a one battle among hundreds, one cast among thousands, it goes unnoticed.
And then, when it screws you, wether because you needed it to proc or because you wanted to avoid being eaten by your ennemy at all cost, there you will notice. There you will remember that luck has betrayed you.