Thanks, I'll have to keep that in mind when suggesting INS in the future.
As for the experiment, you wanted to know how long it would take. This would depend entirely on what type of vehicle you used. A small boat would take many, many hours, thus allowing for a pretty large cumulative error in the INS. However, an aeroplane could be used, which could complete the experiment in 5 to 6 hours, thus reducing the cumulative error to a point where it would not invalidate the experiment.