As other replies mentioned, how big is it compared to your long term backtested drawdowns?
If it has exceeded the expected maxdrawdown by a good margin then you need to shut it off or reduce.
Another rule of thumb is 1.5 times the standard deviation of returns - if the drawdown is deeper than this number then consider switching off or reducing.
If this system is part of a portfolio of uncorrrelated systems then you might just want to keep it going at a reduced allocation as it might be providing smoothing effect for your overall trading plan. I'm a proponent of trading as many uncorrelated systems as you can find, this way an 18% drawdown in a particular system translates to a small drawdown in your overall wealth.
This is how I trade my own strategies, but a different story when I allocate to other managers. My current managers are uncorrelated trading different asset classes but all are drawing down this year ranging from -5% to -20%. I might just fire all of them and trade the assets myself.
My largest drawdown prior to this was 9%.
