Cutting back a container grown aretimisia

[sunnyhills]

Limelight .
Can anyone tell me when it's best to do this and how much to take off ?
Or any advice at all ?

Would be fine to do it now. I would take it back to about 15-20 cm so the new growth will quickly cover the stalks.
As you have discovered it is extremely vigorous and should put on new growth quickly. It will however fill the pot with roots and need to be split, either this autumn when it's dying back, or in the spring.

Oh thank you so much .That's just what I wanted to know - when ,how much .
And thank you for saying about splitting it ,that would not have crossed my mind .Though now you've said ,I guess it's obvious .