What should be the singal duration in time doamin to correctly measure its frequency using FFT
For separating sinusoids (resolution of multiple peaks with clear gaps between them in an FFT magnitude result plot), you normally need an FFT length approximately double the reciprocal of the difference in frequency between the pair that is closest together.  This is roughly double the length over which 2 sinusoids are orthogonal, and is required in order to see the gap between, and to have 2 clear peaks instead of just one fat one in an FFT result plot.  This length increases if a non-rectangular window is used.