The deal is good for a week and ends Sunday, June 1, 2014.
A 2-Topping large pizza normally goes for $14.49 at Domino's, although they have another carryout deal that runs Mondays through Thursday where a 3-Topping large pizza is $7.99 each. So, if you're more of a 2-topping person, it's a $2 savings off of the Monday through Thursday deal.
Unlike some of their previous half-off deals, the $5.99 carryout deal is not "online only" i.e. it's also good for phone and in-person orders.