5 same is acceptable I guess....But I would roll average. Also I think I might measure the period of the waveform as opposed to the high time. If a Proximity sensor is used you will get more reliable repeatability if you measure period as opposed to high or low width. This would be particularly true in the case if the buffer circuit you use has an non-symmetrical delay in one transition direction as opposed to the other. This is almost certainly the case with the opto-coupler. The delta delay may be a small number but could make the detected count be off by one sometimes.

Another reason to roll average the readings is that the scheme where you want to just look for 5 readings the same has a bit of flaw in that the readings on any sampled measurement system can be off as much as +/- 1. If you look for 5 counts the same do plan to put a tolerance on the acceptable value values you are comparinng.

Michael Karas