The answer depends upon a number of factors including the particular stock and its historical price activity, the size of the position and what other positions you carry in the account. In general, however, Portfolio Margin allows for a requirement as low as 15%, assuming you are carrying a diversified portfolio of low volatility securities. For a single stock, the requirement will be a minimum of 30%, but could be higher if the stock has exhibited a high degree of volatility over the past 30 days or has increased in price substantially over the past year. In addition, a concentration charge would apply if the position represented at least 1% of the shares outstanding. Find more information on Portfolio Margin in the IBKR website.
Given all of the above, the simplest way to determine the margin requirement for a given position is to use the Check Margin feature in the TWS. Find information on how to preview an order/check margin impact in the IBKR Knowledge Base.