It depends on your winrate and how much your results swing. The safe roll grows with the square of your standard deviation and shrinks as your winrate rises, so a small edge in a swingy game needs far more buy-ins than a big edge in a steady one.
It is the level most winning regulars treat as safe: roughly one chance in a thousand of losing the whole roll if you keep playing the same game at the same winrate. Set 1% or 5% instead if you can reload.