The answer is very rarely probably. The reason this has come to mind is that I have recently had some coloured contact lenses and been wearing them on nights out and at my parent's family do over Christmas to see if anyone noticed. In fact at my parent's I had one green lens in and one eye without lens so blue/grey. Total number people to notice on me being heterochromic - none.
Admittedly celebrities often disguise it with contact lenses but did you know that Benedict Cumberbatch, Mila Kunis and Simon Pegg all have different coloured eyes.
I would like to say I buck this trend but that would be a lie, I couldn't tell you the eye colour of some of my best friends or my wife and I failed to spot that one person I deal with regularly in the student's union has heterochromia until she told me (I then told her what it was called).
The colour of your eyes is determined by a complex interaction between the pigment melanin and other components of the iris including haemoglobin in the blood vessels and collagen in the stroma and vessels. The full method is not understood and neither is the method of inheritance.