This comes up all the time. Do as David Brown suggested and make a new test print.

If you like math the equation is:

new_time = old_time x (new_M +1)^2 / (old_M+1)^2

where M = new magnification (print/neg) and m = old magnification (print/neg)

The exposure time factor would be:

Factor = (M + 1)^2 / (m + 1)^2

The equation does follow the inverse square law for light but, since you re-focus the image, you also have to account for the change in your 'effective aperture.' Similar to 'bellows factors' used in macro photography. When doubling the size of the print, you will be about one stop off if you use the 'inverse square' equation alone.