Pruning eucalyptus help please

I have a 6ft young eucalyptus I planted about 18 months ago, it seems very strong as it took a battering from the winds and is still straight... when do I cut it back and how much please?

 

I have a Eucalyptus(Rubra) in a pot around six feet high as well Jay. I cut nearly every branch back by 1 foot or so but I could have been more ruthless. As I took the top out last summer it branched freely so this year it will branch even more. They will grow quite a bit between now and the autumn so even if you cut it back hard it will grow quickly. Hope this helps.