Largest possible downsampling factor
There is not one correct answer to this question, other than the theoretical one which would be that the maximum downsampling factor $M$ is given by

$$M=\frac{f_s}{f_c}$$

where $f_c$ is the highest frequency of the (ideally band-limited) signal. Since there's no ideally band-limited signal, it depends on the application how much aliasing can be tolerated. The frequency $f_c$ must be chosen as the frequency above which the signal (after low pass filtering) has no significant energy. What 'significant' means is determined by the application.