It depends entirely on what configuration of RADAR you've got, and what you're pointing it at. You can build a system that will result in returns with beat frequencies of just about anything, depending on the Tx modulation and the range. The question is whether it will be useful for your application in terms of range/velocity resolution, latency, integration time etc.
Like I said I was just speculating about why OP specifically mentioned 10-100ms. the light does indeed travel pretty quickly (although, as anybody in the radar/lidar industry will tell you, not nearly quickly enough!), however the round trip time is just the minimum latency you have to eat to get any information about your target. Once you have light coming back, you need to integrate for some about of time to achieve your desired SNR. That time could be very small, or it could be infinite if there are no photons coming back. Let's randomly say that you're using a RADAR with a Tx bandwidth situated such that the round trip time is 1us, and that your target range is s.t. the beat frequency of the return is 1kHz. Your job is to estimate that frequency, so you have to observe the waveform (by integrating samples for an FFT, typically) for at least one cycle of the RF wavelength. That would require that you wait 1us for the light to fly, and then wait another 1ms for the RF to cycle once. So your measurement latency is ~1ms. Now that's not 100ms, but perhaps you need more than one cycle to give a good estimate of the frequency, and then even more because the target is faint and there aren't many photons coming back. You could possibly arrive at some much higher number, like 10-100ms.
I'm not sure if that was OP's point, but that's all I'm saying ;-)
Yeah, it definitely can be shorter than 100ms. See my sibling comment. It just depends on the type of radar being used and the target range/velocity. Certainly for shortish range targets and mmwave radars on reasonably reflective targets you can get a signal with decent SNR in shorter time frames.
