Best done around May, when the growing season's underway and its growing strongly. Best done in a considered fashion, standing back and working out which stems to lop out, because if you just take the height down and prune round the edges, you end up with lots of 'stumps' to look at for some weeks, so have a good look at it first, and if its got lots of stems coming up from the ground, decide which ones to take out completely at ground level, then reduce the topgrowth on the others if you have to. Bear in mind these are large shrubs though - they do want to get up to 10 feet high and around 6-8 feet wide.

If it is more than a meter or so taller than your target height, You might want to take it down in stages:
Take out 1/4 to 1/3 of the tallest stems to ground level, or to short side stems near ground level.
Next year, take out another fraction of stems.
Repeat until the plant is near the height that you want, then just thin out a stem or two as needed to maintain it.
As Bamboo says, in the long run, it may be better to move the Fatsia to a larger home, and choose a better shrub for this shrub.