What is such parameter called for a frequency to voltage converter
Page 8 of the data sheet shows the 0-99% step response as &lt;35mS, when the input frequency is above 500Hz. This means it's undefined below 500Hz, which is reasonable, as presumably the detector needs several cycles to settle, and so the response time is likely to increase.

This may or may not be exactly what you're after. Other definitions of a similar thing would be the latency (usually the time between the 50% point on the input change and the 50% point on the output change), the frequency response (be careful as the input is also a frequency, to not get confused), and the time constant (making an assumption that the response is first order). 

Note that step responses are usually only specified to 90% of the final change, it's easier to measure that point with an oscilloscope than the 99% point. It sounds like they're trying to use 99% to mean 'fully settled', which it more or less is.