Don't know if they have changed, but back in 2008 for me it was near the end of RTH. They wanted to clean you up before the change to overnight margin. But like d08 said when they do it probably depends on how far under you are.
According to this IB page https://www.interactivebrokers.com/en/index.php?f=24862 do they calculate your margin status at the following times: If your SMA becomes negative will IB (partially) close positions until your account gets within the allowable margin level (i.e. until SMA becomes positive again). These position adjustments are done before the market closes.
Got one of these in Dec. Wonder if that took effect? Change to Risk Management and Margin Policies Dear Client, In response to extreme market volatility and substantially increased risk sentiment in global markets, we are taking the following steps with immediate effect: Intraday margin discounts remain suspended until further notice. Liquidation deferrals: the buffer for activation of liquidations is currently 5%, but may be reduced further as market conditions warrant [Background: under normal market conditions IBKR usually delays liquidations for a period of time during the active trading session when an account is only slightly (less than 10%) under its financial requirements to allow clients to individually manage their risk reduction activities]. We will be deploying re-assessments of margin requirements more frequently and in some cases, on an intraday basis. Clients with positions having exposure to large moves in most asset classes (equity, energies, capital markets, forex, bonds) may expect higher capital requirements on short notice and should consider managing their risk and capital positions in their accounts to anticipate such changes. Interactive Brokers Client Services