How common is giant papillary conjunctivitis in patients who do not wear contacts?

Medium. Can be common in contact lens wear otherwise, may happen with allergic or vernal keratoconjunctivitis.
Quite common. In areas with high environmental allergic potential - chronic allergies and vernal conjunctivitis produce giant papules without contact lens use. Of course, most cases are contact lens related and often improve with antihistamines and hydrogen peroxide disinfection regimens.