How high up in the air should i ideally place my hepa air purifier in my apartment? (i have allergy's)

At the nose level?: There is no study I am aware of on this issue. The most part is the amount of air filtered per hour (4-5 exchanges ). I would only advise this for the bedroom when patients have pet allergies yet insisting on keeping a pet. Overall, I am not that impressed with HEPA filter although some people do derive benefit from it. You'd be better off consulting an allergist.