Rewrite equation to solve for t:

I=1.5-1.5(10^(-0.03t))

sorry the upper-case I looks like a 1. . .

i/1.5 = 1-10^(-.03t)
10^(-.03t) = 1 - i/1.5
-.03t = log(1 - i/1.5)
t = -log(1 - i/1.5)/.03