Depends whether you want the flowers and fruit - if so, reduce half the newer stems by half, and cut the older ones down to the ground - best done in midwinter, while its dormant. If you're growing it in a small space and don't have room for it, you can cut it to the ground in winter - it'll be fine, but you'll lose the flowers the following summer. Despite these instructions, I prune back in about July, removing some of the longer lengths, because it starts to look tatty and gets quite leggy.

I find cutting the tips produces horizontal growth, cut out some of the larger branches to ground level in winter. The flowers are up to 18inches across, but no fruit yet.