Sorry for the delayed reply, I've been out earning my living!
To answer the question, almost none - .ooooo1%
I can't remember ever being forced to do a manual update in anger, and only ever did a couple just "for the hell of it". (And with the destination more or less in sight, in case we stuffed it up!!
)
A further point that might help you get your head around it - as BIK 116.8 says, an INS consists of 2 parts, the inertial reference platform and the nav. computer. The reference platform cannot be updated in flight - the aircraft needs to be perfectly stationary for the platform to align. Once it is aligned, it's accelerometers and gyros detect rate of change of movement (ie accleration) and the direction of this change. This info is then fed to the computer part of the INS, which integrates it with time to produce the distance the aircraft has moved in each direction. When you initially aligned the INS you gave it it's position in lat/long, so the computer knew where it was before it started to move - using the distance and direction moved the computer can keep track of where it has moved to.
So, when you update the INS you are actually only updating the nav computer part of the system - you are merely telling it that instead of being where it thinks it is, it is actually soemwhere else
It then carries on using the outputs from the inertial platform to update this new position.
Don't forget that an IRS is merely an inertial platform, it does not contain a nav. computer, which is why you can never update an IRS postion, you can only align it when stationary.
Hope that all makes sense - my body is at 0 west, but my brain is somewhere else!!