Autocorrelation function data points

Dear All,

To calculate the autocorrelation function of the radius of gyration, what is the minimum number of data points I need to have?
Also, I am following an annealing schedule in my MD protocol and I want to eliminate high-temperature radius gyrations and only consider room-temperature ones. In this case, can I use the autocorrelation function calculation of the radius of gyration by skipping some time intervals in the trajectory?

I appreciate your help.

Thank you.