It depends on what type of clematis you have.If you give a specific name then someone can tell you when.

If it is a newly planted one, you should hard prune it the first two years after planting no matter what variety it is. This helps it get off to a great start with a bigger root system and more stems coming up from the ground.

Do it now.

I did not do this with my first Clems years ago and have done it with all newer ones. It makes a huge difference. I went back and hard pruned all of the older scraggly Clems even those that are "no prune" and all of them responded positively. Some has one or two scraggly stems and now have dozens. Some bloomed five or six blooms and now have hundreds of buds.