Credit histories go as far back as 7 years, so in terms of 3 and 6 months, it is a relatively short time. This is also why it is a challenge for many young adults to receive fair credit scores.

Also, you should at least be charging one thing per credit card so they are still remaining active.

As dumb as it may sound, it is actually better for your credit score if you have around $5-$10 on your statement on any given month which achieves a higher credit score vs. paying it off entirely. This is illogical because most smart people just set it up for auto pay and pay it off entirely. Hope that helps.

The shortest I've seen is 6 months. I don't have tons of finance experience though, other banks may do it differently.