When the RSI oscillator enters the Overbought area you should consider selling.
Important : You need to take into consideration more factors when deciding if to buy/sell, not just the oversold/overbought RSI areas. In strong trends, the RSI oscillator might not even enter the oversold/overbought areas for prolonged periods of time, so keep that in mind when making decisions.
Some traders assign different values to the oversold/overbought areas to better fit their trading strategies. I am trading using an oversold area from 0 to 20 and an overbought area from 80 to 100.