How long do I have to quarantine Rosy Minnows?

I love these fish and have a 15 gallon tank with 6 neon tetras. I want to add 5 or 6 rosy minnows but they are sold as feeder fish and I do not want to infect my neons. I have a 2.5 gallon tank for quarantining these fish. If I put a filter that I currently have running in a 20 gallon I have (have extra filters in my tanks just encase I need one with good bacteria) would this keep the water clean enough? How long would I need to do this before placing them with my neons in the 15 gallon? I'm hoping 2 weeks or less as my LFT only accepts returns 15 days after the purchase date.