This is not recommended way of operation and is not intended either. However, if someone wants to know if the input reference clock is suddenly stopped (0MHz), how many cycles of CLKOUT will keep as the former output clock, then the output changes to unlock, it was measured to see how the CY2305 behaves under nominal conditions (25C and 3.3V).
The output usually holds the original frequency for only a few cycles. Generally you cannot expect more than 2 or 3 cycles at the normal frequency. It then drops towards a lower frequency. For example,
100 MHz drops towards 80 MHz, stops after ~120 cycles
80 MHz drops towards 70 MHz, stops after ~120 cycles
60 MHz drops towards 50 MHz, stops after ~90 cycles
40 MHz drops towards 30 MHz, stops after ~50 cycles
20 MHz drops towards 14 MHz, stops after ~25 cycles
10 MHz goes very quickly towards 5 MHz, stops after ~10 cycles
In all cases, the output clock stops in about 1.5 to 2 microseconds.