How long to leave quarantine tank alone?

[ponokareefer]

I had a sick fish in my quarantine that ended up dieing. I am not sure what the disease was as there was no marks on the fish, just that it was breathing very rapidly one day, the next day stopped eating, the next day was very lethargic and then was dead on the last day. It had been healthy and eating prior to this.
This seems to be the symptom of a number of diseases, ich, clownfish disease, velvet, and I was wondering how long I should leave my quarantine tank bare until adding something new? I have read from 2 weeks to 3 months.