How much and how long - Campden?

Last year I apparently overdosed my first cider with Campden. I used 5 tablets and was told by my LHBS (NB) that I had overdone it. They recommended using a venting wand to remove some of the Campden and pitch again. It took off after that, but was sulfurous. The tablets I have read “USG 150PPM Total SO2. Approximately 550mg active Potassium Metabisulphite/tablet.”

Additionally, after I add Campden, how long do I let it do it’s thing before pitching? Do I let it vent/leave it open, or do I put an airlock or seal it completely?

Thanks in advance!

I left mine for 24 hours, which was probably overkill.

24hrs and let it vent.

Thanks. Is a loose cap of foil enough venting, or should I leave it completely open?