Hello you can see from my picture the growth of the branches. My question is when can i start to prune and how aggresive can i be..Thanks

I have never seen a Kilmarnock willow grow in that haphazard fashion. Yes, you're right about the height, more or less, and yes they do spread sideways, but I'm not convinced that growth is actually the Kilmarnock - see if its appreciably different from the other growth on the plant - I'm thinking you may have a reverted shoot or two, or that the growth might be coming from below the graft. If it is either of those, take the rogue growth off right back to the main stem.

Now is quite a suitable time to prune. Stick the prunings in the ground and they will probably root. but only those from the one you want not the root stock

Our Kilmarnock willow has grown in height over the years until it is now approx 6 foot tall - definitely has a larger spread but I've rarely had to prune it.

I forgot to answer your question properly, got carried away with reversion! If the growth turns out to be like the rest, just clip off so that its the same length all round, and prune off the sticky out bits on that side till it looks neater and the same all round.

I did answer that in my final answer previously on 17th July;-)