The nearest expiration date for Brent Crude Oil vs. Dubai Crude Oil (Platts) Futures (Sep 2013) is Oct 1, 2013.
The best time to sell a futures contract is when you've already made money on the investment, but still have plenty of time left before the expiration date. Thus, a good option would be to sell Brent Crude Oil vs. Dubai Crude Oil (Platts) Futures (Sep 2013) before Oct 1, 2013.