There is no such requirement, "technically" or not. If you are getting 60 ignitions per minute, then you can in principle estimate the rotational speed in as little as 1 second.
The minute interval in "per minute" is just a standard reference interval. Alternatively, rotational speeds can be measured and/or expressed in radians per second, cycles per second, degrees per year, etc.
The choice of units does not affect how the rotational speed is measured, and you can easily convert from one set of units to another after the measurement has already been made.