In flywheel energy storage system how long will the flywheel deliver the rated power at rated voltage & frequency once the grid fails? Does it depend on load & inertia of flywheel only? As time passes & it slow down will the voltage and frequency drop?

How long depends upon the load and the amount of energy stored in the flywheel.

Yes the frequency and voltage will drop as the speed of the flywheel decreases unless you have a way to compensate for it.

If the V & Hz drops can't you rectify to DC and invert to required grid voltage/frequency?

Yes you can rectify and invert. You still need to maintain voltage however.

There are losses with both steps.