The RHS say "Wisteria is pruned twice a year, in July or August, then again in January or February.
Summer pruning (July or August)
Cut back the whippy green shoots of the current year’s growth to five or six leaves after flowering in July or August.
This controls the size of the wisteria, preventing it getting into guttering and windows, and encourages it to form flower buds rather than green growth".
Hope this helps you.