It's a very difficult question to answer without knowing what pruning group it's in, but a rough rule of thumb is:
"If it flowers before June - do not prune"
Basically if it flowers in spring and has small flowers, don't prune it. If it flowers in late spring/early summer and has large flowers you should prune it back by about a half, and it should flower later in the year when you can tidy it. If it flowers in mid- to late summer and autumn then it probably needs pruning hard in spring which means roughly back to about 12 to 18 inches.
It sounds as though yours is possibly a group 2 clematis in which case you can prune it back by about a half.
I hope this helps!