As a deciduous tree/shrub I'd prune it in early spring. I believe you can whack it down quite harshly to invigorate it's variegated new growth. I don't personally grow it.

I have two of these in my front yard. One I pruned severly in early spring. This one has filled in nicely and wherever I pruned it has variegated leaves. The other I didn't prunes as much and is scraggly. Mine are growing like weeds in full sun. The nice one is right in front of my porch and would be 8 ft if I let it (I've had it just over a year, was planted by the builder). It got too big a few weeks ago so I pruned it again. I swear it's grown a foot since then. They were widely planted in our subdivision and the ones that have been ignored look terrible. I'm planning to prune it hard this spring and hope for a bushier, more colorful shrub. But I'm new to this and just happy to have something thriving in my yard!

I prune my willow quite harshly in spring, before the leaves start budding out. I pretty much cut off last years growth, check to make sure the large branches are not rubbing anyplace. I have done this the last 4 years.

I want to keep it a managable shrub size, with lots of color. It is a focal point of interest. The spring pruning does a great job. It is amazing what new growth it put out on those bare branches in a very short time. I get new stem growth from a foot to 2 feet by mid-summer. The new leaves on new stems are very white, stay pretty white until about now, when green gets stronger, leaves are larger. I do not trim in summer, only Spring. It does pretty well in a half-shade area, brightens it up.

I did fall planting, then did not prune hard the first year, letting plant establish roots. Color was just willow green, nothing special about shrub at all. Did put on a lot of height, much larger than what I needed. I cut it hard the next Spring, to reduce scraggly growth of branches, better shape, get the white color back. It was like having a totally different shrub that second year.

Sometimes you have to be mean, cut a lot, to get the plant doing what you want. Cut your shrub next Spring, you will like it again.

The color gets better over the first couple of years. Just wait. I wondered about the fuss too but it is outstanding now.
Shape it now, if you want. You can prune this one whenever it needs it.

Heather, is it still just warming up by you? We've had a very cool spring, I think it's safe to prune it a little to shape it. Next year is when you'll really want to cut it back, if I'm interpreting what everyone here is saying correctly.

Maybe you can help me with this question. Do the pink splotches come out more with the new white growth, or when? I saw a standard of this plant, thought it might look cool planted with some hosta and my new hydrangeas, to complement the pink and white blooms and add height to the bed. Or maybe I want just the shrub form? I've also read that the new growth has the best fall color on the stems.