  The Brazil-nut effect is the phenomenon in which a large intruder particle
immersed in a vertically shaken bed of smaller particles rises to the top, even
when it is much denser. The usual practice, while describing these experiments,
has been to use the dimensionless acceleration \Gamma=a \omega^2/g, where a and
\omega are respectively the amplitude and the angular frequency of vibration
and g is the acceleration due to gravity. Considering a vibrated
quasi-two-dimensional bed of mustard seeds, we show here that the peak-to-peak
velocity of shaking v= a\omega, rather than \Gamma, is the relevant parameter
in the regime where boundary-driven granular convection is the main driving
mechanism. We find that the rise-time \tau of an intruder is described by the
scaling law \tau ~ (v-v_c)^{-\alpha}, where v_c is identified as the critical
vibration velocity for the onset of convective motion of the mustard seeds.
This scaling form holds over a wide range of (a,\omega), diameter and density
of the intruder.
