So many variables to account for. Depends on soil, light etc.

I have an established Buddleia davidii, been in four years that puts on more than four foot a year.

I also have some grown from seed, a couple of years old, pot bound - these grew approx eighteen inches last year.

as long as you dont leave it in its little pot, at least double in size.

Get it into a 3L pot now and then a 10L to grow it on some where sheltered before planting out, about June. That way it will grow quicker and establish better.

It should get to a metre plus by the end of the summer, probably more.

Thankyou

I grow this one...it's a really strong grower and the flowers are very beautiful. I find my variegated ones don 't grow quite so fast.