How Long To Quarantine New Fish?

Opinions on this vary widely.

If you just want to quarantine to make sure that the fish don't have something like whitespot, 2 weeks would be fine. For intestinal worms you'd need at least a month, preferably two.
It also depends on what the fish in your main tank are worth. Discus, for example, are expensive fish and it is common to quarantine new fish for two or even three months. But for your barbs, wait till the two week mark is up and they should be OK.