3 times a week for my old horse atm, but he is in rehab (for back problems not drugs
)
When he's in normal work - usually twice a week
I would think for a 'normal' horse as often as you want to so long as you have a good surface, stay on a big circle or preferably an oval, and make frequent changes of rein. Same as riding really.
The only time I would limit lunging is if horse had history of soft tissue problems eg suspensory or tying up.