Wait until all danger of frost is past. I never pruned mine unless there were dead vines, which you can't determine until they leaf out, so I'd say wait until May in your zone. As far as how much, wait and see what leafs out, then cut just what you have to.

Wrong!! The previous response is incorrect in some respects and vague to the extreme in others. Now is the perfect time to prune -- frost has nothing to do with it. Typically for the majority of clematis you prune them the same time you prune roses or when the forsythia is blooming. Some clematis gardeners prune much earlier in the season (December/January) without concerns because that's when they have the time.

How much you prune depends on the type of clematis you have, usually determined by when it blooms and whether on old or new growth or a combination.

There is an entire GW forum on growing clematis and I'd encourage you check it out. You will get some very helpful information from some very knowledgeable posters and pruning is discussed endlessly, especially at this time of year :-)

I'd suggest you look at the recent posted titled "ID question" - this will discuss pruning in detail as well as helping to determine what type of clematis you have and that will dictate how much you should prune.

Here is a link that might be useful: Clematis Forum

Thank you both for the response! I will check out the forum.