Depends on the pedal
For Eggbeaters I unclip if I hit a rock with my pedal which is very rare. For Shimano going around corners, pedaling, going over logs, in short just about anywhere that I am not coasting in a straight line there is always the possibility that I will unclip and it's not because I want to. I have had horrible experiences with Shimano pedals, regardless of cleat position and retention settings I seem to have a knack for blowing my feet out of their pedals. On Speedplays not being clipped in often caused me to 'clip out'. The one time I used Time pedals nothing made me clip out, not even my friend twisting my foot as I lay trapped.