It depends on the medium you grow in. If soil, if it has nutrients built into it (which, usually is a bad idea starting out unless you have a soil that is expected to carry the plant all the way to the end with little help.). Usually people say after the first week. But, if you bought a potting mix like MiracleGro with a lot of nutrients in it, it will be a guessing game when to feed.
Which nutrients did you buy?