The time to reach S/N=10 is a good approximation for the following scenarios:

n=1 => Any R value, Re >= 4

n=2 => R > 2, Re >= 4

n=4 => R > 4, Re >= 4

Due to the sharp variation in Sersic profile at small R, for a better estimate we suggest calculating the time for S/N=10 at R + or - 0.5