MrMark:
I am not sure, what is the minimum time I can get??
The minimum sample rate required is a pretty essential part of specifying any discrete time sampling system. If you're just looking for a fundamental imbalance then twice the rotational rate of your spindle is the lower bound per the Nyquist Theorem. There may be higher order vibrations in a particular system that need be taken into account.
The OP needs to read the analog signal on every interrupt to get the positional resolution required.
So 32 or 64 readings per revolution.
Tom.... 